<PAGE>   1
                                                                     EXHIBIT 4.2


                  CERTIFICATE OF DESIGNATION, VOTING POWERS AND
                        RIGHTS OF SERIES A 6% CONVERTIBLE
                 PREFERRED STOCK OF VISTA ENERGY RESOURCES, INC.


     Vista Energy Resources, Inc., a corporation organized and existing under
the laws of the State of Delaware (the "CORPORATION"), DOES HEREBY CERTIFY:

     That, pursuant to authority conferred upon the Board of Directors by the
Corporation's Certificate of Incorporation, as amended, and pursuant to the
provisions of Section 151 of the Delaware General Corporation Law, as amended,
the Board of Directors, by written consent dated as of February 8, 2000,
unanimously adopted the following resolution providing for the creation and
issuance of a series of shares of the Corporation's authorized preferred stock
designated as "Series A 6% Convertible Preferred Stock":

     RESOLVED, that a new series of the Corporation's authorized but unissued
preferred stock be, and the same hereby is, established and designated, and that
the designation and amount thereof, and the relative rights, preferences,
qualifications, limitations and restrictions thereof, are as follows:

     SECTION 1. DESIGNATION, NUMBER OF SHARES AND STATED VALUE OF SERIES A
PREFERRED STOCK. There is hereby authorized and established a series of the
Corporation's preferred stock, par value $0.01 per share, that shall be
designated as "Series A 6% Convertible Preferred Stock" ("SERIES A PREFERRED"),
and the number of shares constituting such series shall be 5,000,000. Such
number of shares may be increased, but not decreased, by resolution adopted by
the Board of Directors of the Corporation. The "STATED VALUE" per share of
Series A Preferred initially shall be equal to $7.81 (which amount shall be
proportionately adjusted in the case of recapitalizations, stock splits, stock
dividends, combinations of shares or similar matters directly affecting the
Series A Preferred).

     SECTION 2. DEFINITIONS. In addition to the definitions set forth elsewhere
herein, the following terms shall have the meanings indicated:

     "BUSINESS DAY" shall mean any day other than a Saturday, Sunday or a day on
which banking institutions in New York, New York, are authorized or obligated by
law or executive order to close.

     "CLOSING PRICE" with respect to a particular security on any Trading Day
shall mean the last reported sales price, regular way, for such security on such
Trading Day, or, in case no sale takes place on such day, the average of the
closing bid and ask prices, regular way, on such Trading Day, in either case as
reported in the principal consolidated transaction reporting system with respect
to securities listed or admitted to trading on the American Stock Exchange or
the New York Stock Exchange or, if not listed or admitted to trading on the
American Stock Exchange or the New York Stock Exchange, as reported in the
principal consolidated transaction reporting system with respect to securities
listed on the principal national securities exchange on which such security is
listed or

<PAGE>   2
admitted to trading or, if such security is not listed or admitted to trading on
any national securities exchange, the last quoted price as reported by The
Nasdaq Stock Market or such other system then in use.

     "COMMON STOCK" shall mean the common stock, par value $0.01 per share, of
the Corporation.

     "CONVERSION PRICE" shall mean the conversion price per share of Common
Stock into which Series A Preferred is convertible, as such conversion price may
be adjusted pursuant to Section 9 hereof. The initial Conversion Price shall be
$7.81.

     "JUNIOR SECURITIES" means the Common Stock or any other series of stock
issued by the Corporation ranking junior to the Series A Preferred in payment of
dividends or distributions or upon liquidation, dissolution or winding-up of the
Corporation.

     "MARKET PRICE" per share of Common Stock as of any date shall mean (i) if
the Common Stock is listed or admitted to trading on a national securities
exchange or The Nasdaq Stock Market, the average of the daily Closing Prices for
a period of 30 consecutive Trading Days ending on such date, or (ii) if the
Common Stock is not then so listed or admitted, the fair market value per share
of the Common Stock as reasonably determined by an investment banking firm
mutually acceptable to the Corporation and the holders of shares of Series A
Preferred.

     "ORIGINAL ISSUE DATE" shall mean the date on which shares of Series A
Preferred are first issued.

     "PARITY SECURITY" means any class or series of stock issued by the
Corporation ranking on a parity with the Series A Preferred in payment of
dividends or distributions or upon liquidation, dissolution or winding-up of the
Corporation.

     "PAYABLE-IN-KIND" or "PAID-IN-KIND" when used in reference to any dividend
payable on the shares of Series A Preferred means payment of the dividend by
issuance of that number of additional shares of Series A Preferred that has an
aggregate Stated Value equal to the dollar amount of such dividend then payable.
In lieu of any fractional share of Series A Preferred Stock to which a holder of
Series A Preferred Stock would otherwise be entitled, such holder shall receive
the nearest whole share of Series A Preferred Stock (i.e. if less than .5
rounded down, and if .5 or more rounded up). Shares of Series A Preferred issued
as dividends Payable-in-Kind shall be duly authorized, validly issued and
nonassessable and, upon issuance, shall have rights (including without
limitation dividend, voting, conversion and redemption rights) identical to the
outstanding shares of Series A Preferred in respect of which they are issued.

     "PERSON" means any individual, corporation, association, partnership, joint
venture, limited liability company, trust, estate, or other entity or
organization.



                                       -2-
<PAGE>   3
     "SENIOR SECURITIES" means any class or series of stock, or any class or
series of debt or debt security convertible or exchangeable into any class or
series of stock, issued by the Corporation ranking senior to the Series A
Preferred in payment of dividends or distributions or upon liquidation,
dissolution or winding-up of the Corporation.

     "TRADING DAY" with respect to any security means (i) if such security is
listed or admitted for trading on any national securities exchange, a day on
which such national securities exchange is open for trading, or (ii) if such
security is not listed or admitted to trading on any national securities
exchange, a Business Day.

     "UNDERLYING COMMON STOCK" means at any time, with respect to any share of
Series A Preferred, the aggregate number of shares of Common Stock into which
such share is then convertible pursuant to Section 9 hereof.

     SECTION 3. DIVIDENDS AND DISTRIBUTIONS.

          (a) The holders of outstanding shares of Series A Preferred shall be
     entitled to receive, as and when declared by the Corporation, out of funds
     of the Corporation legally available for the payment of dividends,
     preferential quarterly dividends at the times and at the rates provided for
     in this Section 3. Dividends on shares of the Series A Preferred shall be
     cumulative and shall accrue whether or not there shall be (at the time such
     dividend becomes payable or at any other time) profits, surplus or other
     funds of the Corporation legally available for the payment of dividends.

          (b) Dividends shall accrue on each outstanding share of Series A
     Preferred at the rate of six percent (6%) per annum of the Stated Value
     (the "DIVIDEND RATE") of such share. Dividends shall be payable quarterly,
     in arrears, as of the last Business Day of each December, March, June and
     September, commencing with March 2000 (each, a "REGULAR DIVIDEND PAYMENT
     DATE"). Dividends shall also be payable on any Redemption Date and on the
     final distribution date relating to the dissolution, liquidating or winding
     up of the Corporation (each such date, together with each Regular Dividend
     Payment Date, is a "DIVIDEND PAYMENT DATE").

          (c) Dividends on outstanding shares of Series A Preferred shall be
     Payable-in-Kind during the period (the "EXCLUSIVE PIK PERIOD") beginning on
     the Original Issue Date and ending on the earlier of (i) the Dividend
     Payment Date on December 31, 2001, and (ii) the liquidation, dissolution or
     winding up of the Corporation (in connection with the bankruptcy or
     insolvency of the Corporation or otherwise). After the Exclusive PIK
     Period, dividends on the shares of Series A Preferred shall be
     Payable-in-Kind or, at the Corporation's option, payable in cash.

          (d) The amount of dividends payable on each Dividend Payment Date
     shall be determined by applying the Dividend Rate from but excluding the
     immediately preceding



                                       -3-
<PAGE>   4
     Dividend Payment Date (or from but excluding the date of issuance of shares
     of Series A Preferred, with respect to the first dividend period) to and
     including the Dividend Payment Date.

          (e) Notwithstanding the foregoing or anything else herein to the
     contrary, however, dividends payable on any Redemption Date (as defined in
     Section 6(c) hereof) or the final distribution date relating to the
     dissolution, liquidation or winding up of the Corporation, shall be payable
     in cash only. If a Dividend Payment Date does not occur on a Regular
     Dividend Payment Date, dividends shall be calculated on the basis of the
     actual number of days elapsed from but excluding the immediately preceding
     Regular Dividend Payment Date to and including such non-Regular Dividend
     Payment Date. Dividends payable on the shares of Series A Preferred for any
     period of less than a full quarterly dividend period shall be computed on
     the basis of a 360-day year comprised of twelve 30-day months.

          (f) Dividends payable on each Dividend Payment Date shall be paid to
     record holders of the shares of Series A Preferred as they appear on the
     books of the Corporation at the close of business on the 10th Business Day
     immediately preceding the respective Dividend Payment Date or on such other
     record date as may be fixed by the Board of Directors of the Corporation in
     advance of a Dividend Payment Date, provided that no such record date shall
     be less than 10 nor more than 60 calendar days preceding such Dividend
     Payment Date.

          (g) So long as any shares of Series A Preferred are outstanding:

               (i) No dividend or other distribution shall be declared or paid,
          or set apart for payment, on or in respect of, any Junior Securities
          (other than dividends or distributions payable in shares of Junior
          Securities or in rights to purchase Junior Securities), nor shall any
          Junior Securities be redeemed, purchased or otherwise acquired for any
          consideration (or any money be paid to a sinking fund or otherwise set
          apart for the purchase or redemption of any such Junior Securities)
          unless full cumulative dividends on all outstanding shares of Series A
          Preferred and any Parity Securities have been or contemporaneously are
          declared and paid for all dividend periods terminating on or prior to
          the date set for payment of such dividend.

               (ii) No dividend or other distribution, except as described in
          the next succeeding sentence, shall be declared or paid, or set apart
          for payment, on or in respect of, Series A Preferred or any Parity
          Securities for any period unless full cumulative dividends on all
          outstanding shares of Series A Preferred and any Parity Securities
          have been or contemporaneously are declared and paid for all dividend
          periods terminating on or prior to the date set for payment of such
          dividend. When dividends are not paid in full, as aforesaid, on the
          shares of Series A Preferred and any Parity Securities, all dividends
          declared upon such Parity Securities shall be



                                      -4-
<PAGE>   5
          declared and paid pro rata so that the amounts of dividends per share
          declared and paid on the shares of Series A Preferred and such Parity
          Securities shall in all cases bear to each other the same ratio that
          unpaid dividends per share on the Series A Preferred and on such
          Parity Securities bear to each other.

               (iii) No shares of Series A Preferred or any Parity Securities
          shall be redeemed, purchased or otherwise acquired for any
          consideration (or any money be paid to a sinking fund or otherwise set
          apart for the purchase or redemption of any such Parity Security) by
          the Corporation unless the full cumulative dividends on all
          outstanding shares of Series A Preferred shall have been or
          contemporaneously are declared and paid for all dividend periods
          terminating on or prior to the date on which such redemption, purchase
          or other payment is to occur.

               (iv) No dividend or other distribution of cash or debt shall be
          declared or paid, or set apart for payment, on or in respect of, any
          Junior Securities, nor shall any Junior Securities be redeemed,
          purchased or otherwise acquired for any cash or debt consideration (or
          any money or debt paid to a sinking fund or otherwise set apart for
          the purchase or redemption of any Junior Securities) unless full
          cumulative dividends on all outstanding shares of Series A Preferred
          have been or contemporaneously are declared and paid in cash for all
          dividend periods terminating on or prior to the date set for payment
          of such dividend or distribution.

               (v) No dividend or other distribution of cash or debt shall be
          declared or paid, or set apart for payment, on or in respect of, any
          Parity Securities unless full cumulative dividends on all outstanding
          shares of Series A Preferred have been or contemporaneously are
          declared and paid in cash for all dividend periods terminating on or
          prior to the date set for payment of such dividend or distribution. No
          Parity Securities be redeemed, purchased or otherwise acquired for any
          cash or debt consideration (or any money or debt paid to a sinking
          fund or otherwise set apart for the purchase or redemption of any
          Parity Securities) unless the Series A Preferred shall have been
          called for redemption pursuant to Section 6 hereof and the full
          Redemption Price (as defined in Section 6(b) hereof) shall have been
          or contemporaneously is paid in cash on or prior to the date set for
          payment of such dividend or distribution.

     SECTION 4. CERTAIN COVENANTS AND RESTRICTIONS. So long as any shares of
Series A Preferred are outstanding:

          (a) The Corporation shall at all times reserve and keep available for
     issuance upon the conversion of the shares of Series A Preferred as
     provided in Section 6 and Section 9 hereof, respectively, such number of
     its authorized but unissued shares of Common Stock as will be sufficient to
     permit the conversion of all outstanding shares of Series A Preferred and
     all other securities and instruments convertible into shares of Common
     Stock, and shall



                                      -5-
<PAGE>   6

     take all reasonable action within its power required to increase the
     authorized number of shares of Common Stock necessary to permit the
     conversion of all outstanding shares of Series A Preferred and all such
     other securities and instruments convertible into shares of Common Stock.

          (b) The Corporation covenants and agrees that all shares of Common
     Stock that may be issued upon exercise of the conversion rights of shares
     of Series A Preferred will, upon issuance, be duly authorized, valid
     issued, fully-paid and nonassessable.

          (c) Prior to the delivery of any securities which the Corporation
     shall be obligated to deliver upon payment of dividends Payable-in-Kind or
     conversion of the Series A Preferred, the Corporation will endeavor to
     comply with all federal and state securities laws and regulations
     thereunder requiring the registration of such securities with, or any
     approval of or consent to the delivery of such securities by, any
     governmental authority.

          (d) The Corporation shall pay all taxes and other governmental charges
     (other than any income or franchise taxes), documentary stamp or similar
     issue or transfer taxes that may be imposed with respect to the issue or
     delivery of the Series A Preferred (or any other securities issued on
     account of the Series A Preferred pursuant hereto) or shares of Common
     Stock upon conversion of Series A Preferred or payment of dividends
     Payable-in-Kind as provided herein or otherwise. The Corporation shall not
     be required, however, to pay any tax or other charge imposed in connection
     with any transfer involved in the issue of any certificate for shares of
     Common Stock in any name other than that of the registered holder of the
     shares of the Series A Preferred surrendered in connection with the
     conversion thereof or otherwise, and, in such case, the Corporation shall
     not be required to issue or deliver any stock certificate until such tax or
     other charge has been paid, or it has been established to the Corporation's
     satisfaction that no tax or other charge is due.

     SECTION 5. LIQUIDATION PREFERENCE.

          (a) In the event of any liquidation, dissolution or winding-up of the
     Corporation (in connection with the bankruptcy or insolvency of the
     Corporation or otherwise), whether voluntary or involuntary, before any
     payment or distribution of the assets of the Corporation (whether capital
     or surplus) shall be made to or set apart for the holders of shares of any
     Junior Securities, the holders of the shares of Series A Preferred shall be
     entitled to receive an amount per share in cash equal to the Stated Value
     per share held by them, plus an amount in cash equal to the full cumulative
     dividends accrued and unpaid thereon, to the date of such payment, whether
     or not declared. No payment on account of any such liquidation, dissolution
     or winding-up of the Corporation shall be paid to the holders of the shares
     of Series A Preferred or the holders of any Parity Securities unless there
     shall be paid at the same time to the holders of the shares of Series A
     Preferred and the holders of any Parity Securities proportionate amounts
     determined ratably in proportion to the full preferential amounts to which
     the holders of all outstanding shares of Series A Preferred and the holders


                                      -6-
<PAGE>   7

     of all such outstanding Parity Securities are respectively entitled with
     respect to such distribution (but only to the extent of such preferential
     amounts). For purposes of this Section 5, neither a consolidation or merger
     of the Corporation with one or more partnerships, corporations or other
     entities nor a sale, lease, exchange or transfer of all or any substantial
     part of the Corporation's assets for cash, securities or other property
     shall be deemed to be a liquidation, dissolution or winding-up of the
     Corporation, whether voluntary or involuntary.

          (b) After payment of the full amount of the liquidation preference to
     which the holders of shares of Series A Preferred are entitled, such
     holders will not be entitled to any further participation in any
     distribution of assets of the Corporation.

          (c) Written notice of any liquidation, dissolution or winding-up of
     the Corporation, stating the payment date or dates when and the place or
     places where the amounts distributable in such circumstances shall be
     payable, shall be given by first class mail, postage prepaid, not less than
     15 days prior to any payment date stated therein, to the holders of record
     of the shares of Series A Preferred at their respective addresses as the
     same shall appear in the records of the Corporation.

     SECTION 6. REDEMPTION. The outstanding shares of Series A Preferred are
subject to redemption in accordance with the following provisions:

          (a) Subject to the terms hereof, the Corporation may at its option
     elect to redeem outstanding shares of Series A Preferred, in whole or in
     part (pro-rata among the outstanding shares), on any Business Day if both
     (i) the average of the Closing Prices of the Corporation's Common Stock
     during any period of at least 30 consecutive Trading Days during the
     preceding 60 Trading Days shall have exceeded 120% of the Conversion Price,
     and (ii) the Closing Price per share of the Corporation's Common Stock on
     the Trading Day immediately preceding the day on which such notice is given
     shall have equaled or exceeded 110% of the Conversion Price.

          (b) The redemption price per share for Series A Preferred (the
     "REDEMPTION PRICE") shall be equal to the Stated Value plus an amount equal
     to the aggregate dollar amount of all accrued and unpaid dividends through
     the Redemption Date. The Redemption Price shall be paid in cash from any
     source of funds legally available therefor.

          (c) The Business Day designated by the Corporation for the redemption
     of shares of Series A Preferred is referred to herein as the "REDEMPTION
     DATE." Not less than 60 days nor more than 75 days prior to the Redemption
     Date, a notice specifying the time and place of such redemption shall be
     given by first class mail, postage prepaid, to the holders of record of the
     shares of Series A Preferred to be redeemed at their respective addresses
     as the same shall appear on the books of the Corporation (but no failure to
     mail such notice or any defect therein shall affect the validity of the
     proceedings for redemption except as to the

                                      -7-
<PAGE>   8
     holder to whom the Corporation has failed to mail such notice or except as
     to the holder whose notice was defective), calling upon each such holder of
     record to surrender to the Corporation on the Redemption Date at the place
     designated in such notice such holder's certificate or certificates
     representing the then outstanding shares of Series A Preferred held by such
     holder. Each such notice of redemption shall specify the Redemption Date,
     the Redemption Price, the place or places of payment, that payment will be
     made upon presentation and surrender of the shares of Series A Preferred,
     that dividends will cease to accrue on such shares on and after the
     Redemption Date, the then-effective Conversion Price pursuant to Section 9
     hereof, and that the right of holders to convert shares of Series A
     Preferred will terminate at the close of business on the Business Day
     immediately preceding the Redemption Date (unless the Corporation defaults
     in the payment of the Redemption Price). On or after the Redemption Date,
     each holder of shares of Series A Preferred called for redemption shall
     surrender such holder's certificate or certificates for such shares to the
     Corporation at the place designated in the redemption notice and shall
     thereupon be entitled to receive payment of the Redemption Price in the
     manner set forth in Section 6(b) above. If the redemption is delayed for
     any reason, dividends shall continue to accrue on the shares of Series A
     Preferred outstanding, and shall be added to and become a part of the
     Redemption Price of such shares, until the Redemption Price, as so
     adjusted, for such shares is paid in full.

          (d) Notwithstanding the foregoing, if notice of redemption has been
     given pursuant to Sections 6(a) and 6(c) hereof and any holder of the
     Series A Preferred shall, before the close of business on the Business Day
     immediately preceding the Redemption Date, give notice to the Corporation
     pursuant to Section 9(b) hereof of the conversion of any or all of the
     shares to be redeemed held by that holder, then (i) the Corporation shall
     not have the right to redeem those shares for which the conversion notice
     has been given, (ii) the holder shall not be entitled to payment of the
     Redemption Price with respect to those shares, (iii) the conversion of
     those shares shall become effective as provided in Section 9 hereof, and
     (iv) any funds that have been deposited for the payment of the Redemption
     Price of those shares shall be returned to the Corporation immediately
     after such conversion.

     SECTION 7. SHARES TO BE RETIRED. All shares of Series A Preferred
repurchased, redeemed, converted or otherwise acquired by the Corporation shall
be retired and cancelled and shall be restored to the status of authorized but
unissued shares of Preferred Stock, without designation as to series, and may
thereafter be reissued (subject to the provisions of Section 8 hereof).

     SECTION 8. VOTING RIGHTS.

          (a) Except as otherwise provided in this Section 8 or required by law
     or any provision of the Certificate of Incorporation of the Corporation,
     the holders of the shares of Series A Preferred shall vote together with
     the shares of Common Stock as a single class at any annual or special
     meeting of stockholders of the Corporation upon the following basis:



                                      -8-
<PAGE>   9

     each holder of shares of Series A Preferred shall be entitled to such
     number of votes for the shares of Series A Preferred held by such holder on
     the record date fixed for such meeting as shall be equal to the whole
     number of shares of Underlying Common Stock with respect to such shares of
     Series A Preferred immediately after the close of business on the record
     date fixed for such meeting. In all cases where the holders of separate
     classes or series of the Corporation's capital stock have the right to vote
     separately as a class, the holders of Series A Preferred shall be entitled
     to one vote for each such share held by them.

          (b) For so long as any shares of Series A Preferred remain
     outstanding, the Corporation shall not, without the affirmative vote or
     consent of the holders of a majority of the shares of Series A Preferred
     voting together as a single class, authorize, create or issue, or increase
     the authorized or issued amount of, any class or series of stock of Senior
     Securities, Parity Securities or Series A Preferred (except through payment
     of dividends Payable-in-Kind on the Series A Preferred), or any security
     convertible into or exchangeable for Senior Securities or Parity Securities
     or reclassify or modify any Junior Securities so as to become Parity
     Securities or Senior Securities.

          (c) Shares of Series A Preferred acquired by a subsidiary of the
     Corporation shall have no voting rights.

          (d) For so long as any shares of Series A Preferred remain
     outstanding, the Corporation shall not, without the affirmative vote or
     consent of the majority (unless a higher percentage shall then be specified
     by law) of the shares of Series A Preferred voting together as a single
     class, merge or consolidate with or into any other Person or permit any
     other Person to merge with or into the Corporation, unless each share of
     Series A Preferred shall remain outstanding and unaffected or shall be
     converted into or exchanged for convertible preferred stock of the
     surviving entity (and, in the case of triangular mergers, with conversion
     rights into the common equity of the ultimate parent entity) having powers,
     preferences and relative participating, optional and other rights, and
     qualifications, limitations and restrictions thereof identical to a share
     of Series A Preferred, and unless the affirmative vote or consent of the
     holders of all the shares of Series A Preferred voting together as a single
     class shall have been obtained if such exchange, merger or consolidation
     shall result in the existence of Senior Securities or Parity Securities
     (treating securities issued by the surviving entity as having been issued
     by the Corporation) other than those previously approved pursuant to
     Section 8(b) hereof; provided, however, that the voting rights with respect
     to this Section 8(d) shall be exercisable only by the original holder of
     the Series A Preferred and such voting rights shall terminate on the first
     date on which such original holder no longer owns shares of Series A
     Preferred which constitute at least 14.03% of the total issued and
     outstanding shares of Common Stock of the Corporation (based upon the
     number of shares of Common Stock into which the shares of Series A
     Preferred are then convertible and deeming the shares of Series A Preferred
     to be converted into issued and outstanding shares of Common Stock for the
     purpose of such computation).


                                     -9-
<PAGE>   10
     SECTION 9. CONVERSION RIGHTS. Holders of shares of Series A Preferred shall
have the right to convert all or a portion of such shares into shares of Common
Stock, as follows:

          (a) Subject to and upon compliance with the provisions of this Section
     9, shares of Series A Preferred shall be convertible at the option of the
     holder thereof into duly authorized, validly issued, fully paid and
     non-assessable shares of Common Stock. The number of shares of Common Stock
     deliverable upon conversion of one share of Series A Preferred shall be
     determined by dividing the Stated Value of such share of Series A Preferred
     by the Conversion Price then in effect (i.e., prior to any adjustment of
     the Conversion Price, one share of Common Stock will be issued for each
     share of Series A Preferred surrendered for conversion). In lieu of any
     fractional share of Common Stock to which a holder of Series A Preferred
     Stock would otherwise be entitled, such holder shall receive the nearest
     whole share of Common Stock (i.e., if less than .5 rounded down, and if .5
     or more rounded up). A holder of Series A Preferred may provisionally
     convert any shares of Series A Preferred that are the subject of a pending
     registration with the Securities and Exchange Commission. Unless the holder
     states an earlier time in the conversion notice to the Corporation, a
     provisional conversion will be effective only as of the closing of the
     holder's sale of Common Stock issuable upon conversion of Series A
     Preferred made pursuant to such an effective registration statement (and
     only with respect to the shares so sold); otherwise, such shares of Series
     A Preferred shall be outstanding as Series A Preferred for all purposes
     hereunder (including accrual and payment of dividends) during any period in
     which a provisional conversion is pending.

          (b) The conversion of shares of Series A Preferred may be effected by
     the holder thereof by the surrender of the certificate for such shares to
     the Corporation at the principal office of the Corporation. If the
     conversion is to be provisional as provided in Section 9(a) above, the
     holder shall also deliver a notice to the Corporation with a statement to
     that effect. If any shares of Series A Preferred are called for redemption
     pursuant to Section 6 hereof, such right of conversion shall cease and
     terminate as to the shares called for redemption at the close of business
     on the Business Day immediately preceding the Redemption Date, unless the
     Corporation shall default in the payment of the Redemption Price, in which
     event such conversion right shall remain in effect until full payment of
     the Redemption Price has been made.

          (c) As promptly as practicable after the surrender of shares of Series
     A Preferred for conversion (unless the shares are surrendered in a
     provisional conversion as provided in Section 9(a) hereof), the Corporation
     shall issue and deliver or cause to be issued and delivered to the holder
     of such shares certificates representing the number of fully paid and
     non-assessable shares of Common Stock into which such shares of Series A
     Preferred have been converted in accordance with the provisions of this
     Section 9. In lieu of any fractional share of Common Stock to which a
     holder of Series A Preferred Stock would otherwise be entitled, such holder
     shall receive the nearest whole share of Common Stock (i.e., if less than
     .5 rounded down, and if .5 or more rounded up). Subject to the following
     provisions of this


                                      -10-
<PAGE>   11

     Section 9, and, except with respect to provisional conversions, such
     conversion shall be deemed to have been made as of the close of business on
     the date on which the shares of Series A Preferred shall have been
     surrendered for conversion in the manner herein provided, so that all
     rights of the holder of the shares of Series A Preferred so surrendered
     shall cease at such time, and the person or persons entitled to receive the
     shares of Common Stock upon conversion thereof shall be treated for all
     purposes as having become the record holder or holders of such shares of
     Common Stock at such time; provided, however, that any such surrender
     (other than for provisional conversion) on any date when the stock transfer
     books of the Corporation are closed shall be deemed to have been made, and
     shall be effective to terminate the rights of the holder or holders of the
     shares of Series A Preferred so surrendered for conversion and to
     constitute the person or persons entitled to receive such shares of Common
     Stock as the record holder or holders thereof for all purposes, at the
     opening of business on the next succeeding day on which such transfer books
     are open and such conversion shall be at the Conversion Price in effect at
     such time. If shares are surrendered for provisional conversion, the
     Corporation shall either (i) reissue certificates for the number of shares
     of Series A Preferred surrendered for provisional conversion, if the sale
     of the underlying Common Stock (as provided in Section 9(a) above) does not
     occur, or (ii) issue certificates for the number of shares of Common Stock
     into which the shares provisionally surrendered have been converted to the
     order of the purchaser thereof, if the sale of the underlying Common Stock
     (as provided in Section 9(a) above) does occur. If, following the delivery
     of a notice of redemption by the Corporation pursuant to Section 6 hereof,
     but not otherwise, a holder of shares of Series A Preferred elects to
     exercise its rights of conversion under this Section 9, such holders of
     shares of Series A Preferred who surrender their shares of Series A
     Preferred for conversion shall be entitled to payment in cash by the
     Corporation of all accrued and unpaid dividends to the date the Series A
     Preferred are surrendered for conversion (or deemed to have been converted,
     in the case of shares provisionally converted), prorated for any portion of
     the then current dividend period during which the Series A Preferred are
     surrendered for conversion, whether or not any dividend in respect of the
     then-current dividend period has been declared or set aside. The
     Corporation shall make that payment as promptly as practicable following
     the surrender of certificates for shares of Series A Preferred for
     conversion as provided herein (or following the sale of the underlying
     Common Stock, in the case of shares that are provisionally converted and
     subsequently sold as provided in Section 9(a) hereof) and, when applicable,
     shall accompany the certificates for Common Stock into which such shares of
     Series A Preferred have been converted.

          (d) Before taking any action which would cause an adjustment reducing
     the Conversion Price below the then par value of the shares of Common Stock
     deliverable upon conversion of the shares of Series A Preferred, the
     Corporation will take any corporate action which may, in the opinion of its
     counsel, be necessary in order that the Corporation may validly and legally
     issue fully paid and non-assessable shares of Common Stock at such adjusted
     Conversion Price.


                                      -11-
<PAGE>   12

          (e) The Conversion Price shall be subject to adjustment from time to
     time as follows:

               (i) In case at any time the Corporation shall (A) subdivide the
          outstanding shares of Common Stock into a greater number of shares, or
          (B) combine the outstanding shares of Common Stock into a smaller
          number of shares, the Conversion Price in effect immediately prior
          thereto shall be adjusted proportionately so that the adjusted
          Conversion Price shall bear the same relation to the Conversion Price
          in effect immediately prior to such event as the total number of
          shares of Common Stock outstanding immediately prior to such event
          shall bear to the total number of shares of Common Stock outstanding
          immediately after such event. Such adjustment shall become effective
          immediately after the effective date of a subdivision or combination.

               (ii) In case at any time the Corporation shall declare, order,
          pay or make any dividend or other distribution to holders of the
          Common Stock payable in shares of Common Stock, then, in each such
          case, subject to Section 9(e)(v) hereof, the Conversion Price in
          effect immediately prior to the close of business on the record date
          fixed for determination of holders of any class of securities entitled
          to receive such dividend or distribution shall be reduced to a price
          (calculated to the nearest .001 of a cent) determined by multiplying
          such Conversion Price by a fraction:

                    (A) the numerator of which shall be the number of shares of
               Common Stock outstanding immediately prior to such dividend or
               distribution; and

                    (B) the denominator of which shall be the number of shares
               of Common Stock outstanding immediately after such dividend or
               distribution.

          Shares of Common Stock owned by or held for the account of the
          Corporation shall not be deemed outstanding for the purpose of any
          such computation. Such adjustment shall be made on the date such
          dividend is paid or such distribution is made and shall become
          effective retroactive to the record date for the determination of
          stockholders entitled to receive such dividend or distribution.

               (iii) In case at any time the Corporation shall declare, order,
          pay or make any dividend or other distribution to all holders of the
          Common Stock, other than a dividend payable in shares of Common Stock
          (including, without limitation, dividends or distributions payable in
          cash, evidences of indebtedness, rights, options or warrants to
          subscribe or purchase any Common Stock or other securities, or any
          other securities or other property, but excluding any rights to
          purchase any stock or other securities if such rights are not
          separable from the Common Stock except upon the occurrence of a
          contingency beyond the control of the Corporation), then, and in

                                      -12-
<PAGE>   13
          each such case, subject to Section 9(e)(v) hereof, the Conversion
          Price in effect immediately prior to the close of business on the
          record date fixed for the determination of holders of Common Stock
          entitled to receive such dividend or distribution shall be reduced to
          a price (calculated to the nearest .001 of a cent) determined by
          multiplying such Conversion Price by a fraction:

                    (A) the numerator of which shall be the Market Price per
               share of Common Stock in effect as of such record date or, if the
               Common Stock trades on an ex-dividend basis, on the Trading Day
               immediately prior to the date of commencement of ex-dividend
               trading, less the value of such dividend or distribution (as
               determined in good faith by the Board of Directors of the
               Corporation) applicable to one share of Common Stock; and

                    (B) the denominator of which shall be such Market Price per
               share of Common Stock as of such record date or, if the Capital
               Stock trades on an ex-dividend basis, on the Trading Day
               immediately prior to the date of commencement of ex-dividend
               trading.

          Such adjustment shall be made on the date such dividend is paid or
          such distribution is made and shall become effective retroactive to
          the record date for the determination of stockholders entitled to
          receive such dividend or distribution. In case at any time the
          Corporation shall declare, order, pay or make a dividend or rights to
          purchase any stock or other securities that are not separable from the
          Common Stock except upon the occurrence of a contingency beyond the
          control of the Corporation, the Corporation shall make provision so
          that the holder of any share of Series A Preferred thereafter
          surrendered for conversion shall be entitled to receive the number of
          shares of Common Stock and rights to purchase stock or other
          securities that such holder would have owned or have been entitled to
          receive after the occurrence of any of such events had the share of
          Series A Preferred been surrendered for conversion immediately before
          the occurrence of such event or the record date therefor, whichever is
          earlier. If any such rights are redeemed, the holder of Series A
          Preferred shall have the right to receive, in lieu of any such rights,
          any cash, property or securities paid in respect of such redemption.

               (iv) In case at any time the Corporation issues or sells any
          shares of Common Stock or any rights, options or warrants to subscribe
          for or purchase shares of Common Stock or shares having the same
          rights, privileges and preferences as the Common Stock ("EQUIVALENT
          COMMON STOCK") or securities convertible into Common Stock or
          equivalent common stock, at a price per share of Common Stock or
          equivalent common stock (or having a conversion price per share, if a
          security is convertible into shares of Common Stock or equivalent
          common stock) less than the Market Price of the Common Stock as of the
          date of such issue or sale, then upon such issue or sale the
          Conversion Price shall be reduced to such Conversion Price

                                      -13-
<PAGE>   14
          determined by multiplying the Conversion Price in effect immediately
          prior to such issue or sale by a fraction, (y) the numerator of which
          shall be the sum of the number of shares of Common Stock outstanding
          immediately prior to such issue or sale plus the number of shares of
          Common Stock which the aggregate offering price of the total number of
          shares of Common Stock and/or equivalent common stock so to be offered
          (and/or the aggregate initial conversion price of the convertible
          securities so to be offered) would purchase at such Market Price and
          (z) the denominator of which shall be the sum of the number of shares
          of Common Stock outstanding immediately prior to such issue or sale
          plus the number of additional shares of Common Stock and/or equivalent
          common stock to be offered for subscription or purchase (or into which
          the convertible securities so to be offered are initially
          convertible). In case such subscription price may be paid in a
          consideration part of or all of which shall be in a form other than
          cash, the value of such consideration shall be determined in good
          faith by the Board of Directors of the Corporation. Shares of Common
          Stock owned by or held for the account of the Corporation shall not be
          deemed outstanding for the purpose of any such computation. Such issue
          or sale adjustment shall be made successively upon the issuance or
          sale of shares of Common Stock or equivalent common stock or any
          rights, options or warrants to subscribe for or purchase Common Stock
          or equivalent common stock or securities convertible into common stock
          or equivalent common stock. Notwithstanding the foregoing, no
          adjustment of the Conversion Price pursuant to this Section 9(e)(iv)
          shall be made upon (A) the conversion or redemption of shares of
          Series A Preferred; (B) the payment of any stock dividend on the
          Series A Preferred; (C) the issuance of options to officers, directors
          and employees of the Corporation and its subsidiaries to purchase
          shares of Common Stock, including any such options as are issued and
          outstanding as of the Original Issue Date, provided that, with respect
          to all such options, the number of shares of Common Stock that may be
          subject to such options shall be limited to 15% of the Corporation's
          fully diluted shares of Common Stock based upon securities outstanding
          as of the Original Issue Date; (D) the issuance and sale of Common
          Stock upon exercise of any rights or options referenced in the
          immediately preceding clause (C) or in Section 9(e)(iii) above; or (E)
          the issuance and sale of Common Stock in an underwritten public
          offering.

               (v) If the amount of any adjustment of the Conversion Price
          required pursuant to this Section 9(e) would be less than 1% of the
          Conversion Price in effect at the time such adjustment is otherwise so
          required to be made, such amount shall be carried forward and an
          adjustment with respect thereto made at the time of and together with
          any subsequent adjustment which, together with such amount and any
          other amount or amounts so carried forward, shall aggregate at least
          1% of such Conversion Price. All calculations under this Section 9(e)
          shall be made to the nearest .001 of a cent.

               (vi) Except as herein otherwise expressly provided, for all
          purposes of this

                                      -14-
<PAGE>   15
               Section 9(e) the term "Common Stock" shall mean the Common Stock
               and any shares of stock or other class of capital stock of the
               Corporation which is not preferred as to dividends or assets over
               any other class of capital stock of the Corporation and which is
               not subject to redemption, or which is issued to the holders of
               shares of Common Stock upon any reclassification thereof.

          Notwithstanding anything in this Section 9(e), there will be no
          adjustment hereunder as a result of the reverse stock split
          contemplated in Section 5.17 of the Agreement and Plan of Merger dated
          October 8, 1999, to which the Corporation is a party, which reverse
          stock split shall be deemed to have occurred prior to effectiveness of
          this Certificate.

               (f) In case at any time after the Original Issue Date, the
          Corporation shall be a party to any transaction (including without
          limitation a merger, consolidation, statutory share exchange, sale of
          all or substantially all of the Corporation's assets or
          recapitalization of the Common Stock), in each case as a result of
          which shares of Common Stock (or any other securities of the
          Corporation then issuable upon conversion of the Series A Preferred)
          shall be converted into the right to receive stock, securities or
          other property (including without limitation cash or any combination
          thereof) (each of the foregoing transactions being referred to as a
          "FUNDAMENTAL CHANGE TRANSACTION"), then the shares of Series A
          Preferred remaining outstanding will thereafter no longer be subject
          to conversion into Common Stock (or such other securities) pursuant to
          this Section 9, but instead each share shall be convertible into the
          kind and amount of stock and other securities and property receivable
          (including without limitation cash) upon the consummation of such
          Fundamental Change Transaction by a holder of that number of shares or
          fraction thereof of Common Stock (or such other securities) into which
          one share of Series A Preferred was convertible immediately prior to
          such Fundamental Change Transaction assuming such holder of Common
          Stock failed to exercise any right of election as to the kind of
          consideration to be received in such Fundamental Change Transaction.
          The Corporation shall not be a party to any Fundamental Change
          Transaction after which shares of the Series A Preferred shall remain
          outstanding unless the terms of such Fundamental Change Transaction
          are consistent with the provisions of this Section 9(f), and it shall
          not consent or agree to the occurrence of any such Fundamental Change
          Transaction until the Corporation has entered into an agreement with
          the successor or purchasing entity, as the case may be, for the
          benefit of the holders of the shares of Series A Preferred which will
          contain provisions enabling the holders of shares of the Series A
          Preferred which remain outstanding after such Fundamental Change
          Transaction to convert such shares into the consideration received by
          holders of Common Stock (or any other securities of the Corporation
          then issuable upon conversion of the Series A Preferred) at the
          Conversion Price immediately after such Fundamental Change
          Transaction. In the event that at any time, as a result of an
          adjustment made pursuant to this Section 9, the Series A Preferred
          shall become subject to conversion into any securities other than
          shares of Common Stock, thereafter the number of such other securities
          so issuable upon conversion of the shares of Series A Preferred shall
          be subject to adjustment from time to time in a manner and on terms as
          nearly equivalent as practicable to the provisions with

                                      -15-
<PAGE>   16

          respect to the shares of Series A Preferred contained in this Section
          9. The provisions of this Section 9(f) shall similarly apply to
          successive Fundamental Change Transactions.

               (g) Upon the occurrence of any event requiring an adjustment of
          the Conversion Price, then, and in any such case, the Corporation
          shall promptly deliver to the holders of shares of Series A Preferred
          a notice stating the Conversion Price resulting from such adjustment
          and the method of calculation thereof and setting forth a brief
          statement of the facts requiring such adjustment and upon which such
          adjustment is based.

               (h) In case at any time:

                    (i) the Corporation shall declare or pay to all holders of
               Common Stock any dividend (whether payable in Common Stock, cash,
               securities or other property);

                    (ii) there shall be any capital reorganization, or
               reclassification of the Common Stock of the Corporation or
               consolidation or merger of the Corporation with, or sale of all
               or substantially all of its assets to, another corporation or
               other entity;

                    (iii) there shall be a voluntary or involuntary dissolution,
               liquidation or winding-up of the Corporation;

                    (iv) there shall be any other Fundamental Change
               Transaction; or

                    (v) there shall occur any other event that would cause an
               adjustment to the Conversion Price of the Series A Preferred;

          then, in any one or more of such cases, the Corporation shall give to
          the holder of shares of Series A Preferred (A) at least 15 days prior
          to any event referred to in clause (i) or (v) above and at least 30
          days prior to any event referred to in clause (ii), (iii) or (iv)
          above, written notice of the date on which the books of the
          Corporation shall close or records shall be taken for such dividend or
          distribution or for determining rights to vote in respect of any such
          organization, reclassification, consolidation, merger, sale,
          dissolution, liquidation, winding-up or Fundamental Change
          Transaction, and (B) in the case of any such reorganization,
          reclassification, consolidation, merger, sale, dissolution,
          liquidation, winding-up or Fundamental Change Transaction known to the
          Corporation, at least 30 days prior written notice of the date, or if
          not then known, a reasonable approximation thereof by the Corporation)
          when the same shall take place. Such notice in accordance with the
          foregoing clause (A) shall also specify, in the case of any such
          dividend or distribution, the date on which such holders of Common
          Stock shall be entitled thereto, and such notice in accordance with
          the foregoing clause (B) shall also specify the date on which such
          holders of Common Stock shall be entitled to exchange their Common
          Stock, securities or other property deliverable upon such
          reorganization, reclassification, consolidation, merger, sale,

                                      -16-
<PAGE>   17

          dissolution, liquidation, winding-up or Fundamental Change
          Transaction, as the case may be.

               (i) Whenever the Conversion Price is adjusted as provided herein,
          the Corporation shall prepare and mail to the record holders of Series
          A Preferred an officer's certificate signed by the Chairman of the
          Board, President, or Chief Financial Officer of the Corporation
          setting forth the Conversion Price after the adjustment, the method of
          calculation thereof, and a brief statement of the facts requiring the
          adjustment and upon which the adjustment is based. If the calculation
          of the adjustment requires a determination by the Corporations' Board
          of Directors, the certificate shall attach and certify to the
          resolution of the Board of Directors relating to the determination.

               (j) All shares of Common Stock issuable upon the conversion set
          forth in this Section 9 shall be duly authorized, validly issued,
          fully-paid and nonassessable.

     SECTION 10. RANKING. Without limiting the definition of Junior Securities,
the following securities and obligations of the Corporation shall rank junior to
the Series A Preferred with respect to the payments required or permitted to be
made to the holders thereof pursuant to their respective governing instruments
and payments required to be made to the holders of the Series A Preferred
pursuant hereto: the shares of Common Stock. No Senior Securities or Parity
Securities are outstanding or authorized on the date hereof or will be
outstanding or authorized on the Original Issue Date.

     SECTION 11. RECORD HOLDERS. The Corporation may deem and treat the record
holder of any shares of Series A Preferred as the true and lawful owner thereof
for all purposes, and the Corporation shall not be affected by any notice to the
contrary.

     SECTION 12. NOTICES. Except as may otherwise be provided by law or provided
for herein, all notices referred to herein shall be in writing, and all notices
hereunder shall be deemed to have been given upon receipt, in the case of a
notice of conversion given to the Corporation as contemplated in Section 9(b)
hereof, or, in all other cases, upon the earlier of receipt of such notice or
three Business Days after the mailing of such notices sent by first-class mail,
postage prepaid, addressed: If to the Corporation, to its principal executive
offices (Attention: Corporate Secretary) or to any agent of the Corporation
designated as permitted hereby; or if to a holder of the Series A Preferred, to
such holder at the address of such holder as listed in the stock record books of
the Corporation (which shall include the records of the Transfer Agent), or to
such other address as the Corporation or holder, as the case may be, shall have
designated by notice similarly given.

     SECTION 13. REGISTRATION OF TRANSFER. Upon the surrender of any certificate
representing the Series A Preferred to the Corporation or its transfer agent,
the Corporation shall, or shall cause its transfer agent to, at the request of
the record holder of such certificate, execute and deliver (at the Corporation's
expense) a new certificate or certificates in exchange therefor, of Series A
Preferred representing in the aggregate the number of shares of Series A
Preferred represented by the


                                      -17-
<PAGE>   18
surrendered certificate. Each such new certificate shall be registered in such
name and shall represent such number of shares of Series A Preferred as is
requested by the holder of the surrendered certificate and shall be
substantially identical in form to the surrendered certificate, and dividends
shall accrue on the Series A Preferred represented by such new certificate from
the date as of which dividends have been fully paid on the Series A Preferred
represented by the surrendered certificate.

     SECTION 14. REPLACEMENT. Upon receipt of evidence satisfactory to the
Corporation (an affidavit of the registered holder shall be satisfactory) of the
ownership and the loss, theft, destruction or mutilation of any certificate
evidencing shares of Series A Preferred, and, in the case of any such loss,
theft or destruction, upon receipt of indemnity reasonably satisfactory to the
Corporation (provided, that, if the holder is a financial institution or other
institutional investor, its own agreement shall be satisfactory), or, in the
case of any such mutilation, upon surrender of such certificate, the Corporation
shall (at its own expense) execute and deliver in lieu of such certificate a new
certificate of like kind representing the number of shares of Series A Preferred
of such class represented by such lost, stolen, destroyed or mutilated
certificate and dated the date of the lost, stolen, destroyed or mutilated
certificate, and dividends shall accrue on the Series A Preferred represented by
such new certificate from the date to which dividends have been fully paid on
the Series A Preferred represented by the lost, stolen, destroyed or mutilated
certificate.

     SECTION 15. SUCCESSORS AND TRANSFEREES. The provisions applicable to shares
of Series A Preferred shall bind and inure to the benefit of and be enforceable
by the Corporation, the respective successors to the Corporation, and by any
record holder of shares of Series A Preferred.

     IN WITNESS WHEREOF, the Corporation has caused this Certificate to be
executed as of the 8th day of February, 2000.

                              VISTA ENERGY RESOURCES, INC.



                              By:/s/ C. Randall Hill
                                 --------------------------
                                 C. Randall Hill
                                 Chairman and Chief Executive Officer



                                      -18-