<PAGE>   1
                                                                    EXHIBIT 10.1

            FIRST AMENDMENT TO AMENDED AND RESTATED CREDIT AGREEMENT


         This First Amendment to Amended and Restated Credit Agreement (this
"First Amendment") is executed as of the 30th day of June, 2000 (the "Effective
Date"), by and among Prize Energy Resources, L.P., a Delaware limited
partnership ("Borrower"), Prize Energy Corp., a Delaware corporation ("Parent"),
Fleet National Bank, successor-in-interest to BankBoston, N.A., a national
banking association, as Administrative Agent ("Administrative Agent"), and the
financial institutions a party hereto as Banks (individually a "Bank" and
collectively "Banks").

                                   WITNESSETH:

         WHEREAS, Borrower, Vista Resources Partners, L.P., a Texas limited
partnership ("Vista LP"), Midland Resources, Inc., a Texas corporation
("Midland"), Parent, Administrative Agent, First Union National Bank, as
Syndication Agent, CIBC Inc., as Documentation Agent, Bank One, Texas, N.A., as
Lead Manager, and Banks are parties to that certain Amended and Restated Credit
Agreement dated as of February 8, 2000 (as amended, the "Credit Agreement")
(unless otherwise defined herein, all terms used herein with their initial
letter capitalized shall have the meaning given such terms in the Credit
Agreement); and

         WHEREAS, pursuant to the Credit Agreement, Banks have made a revolving
credit/term loan to Borrower, Vista LP and Midland; and

         WHEREAS, Parent and Borrower intend (a) to merge Vista LP and Midland
with and into Borrower, with Borrower being the surviving entity (the "Borrower
Merger"), and (b) to consummate certain other Restructuring Transactions (as
hereinafter defined) to simplify the organizational structure of Parent and its
Subsidiaries; and

         WHEREAS, Parent and Borrower have requested that (a) Banks consent to
the Borrower Merger and the other Restructuring Transactions, and (b) in
connection therewith, the Credit Agreement be amended in certain respects; and

         WHEREAS, subject to the terms and conditions set forth herein, Banks
have agreed to Parent's and Borrower's request.

         NOW THEREFORE, for and in consideration of the mutual covenants and
agreements herein contained and other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged and confessed, Parent,
Borrower, Administrative Agent and each Bank hereby agree as follows:

         SECTION 1. Amendments. In reliance on the representations, warranties,
covenants and agreements contained in this First Amendment and subject to the
satisfaction of the conditions precedent set forth in Section 3 hereof, the
Credit Agreement shall be amended effective as of the Effective Date in the
manner provided in this Section 1.


<PAGE>   2


         1.1. Amendment to Definitions. The definitions of "Administrative
Agent," "BankBoston," "Borrower(s)," "BRSI" and "Loan Papers" contained in
Section 2.1 of the Credit Agreement shall be amended to read in full as follows:

                  "Administrative Agent" means Fleet National Bank,
         successor-in-interest to BankBoston, N.A., in its capacity as
         Administrative Agent for Banks hereunder or any successor thereto.

                  "BankBoston" means Fleet National Bank, successor-in-interest
         to BankBoston, N.A., a national banking association, in its capacity as
         a Bank.

                  "Borrower(s)" means (a) prior to the Borrower Mergers, Prize
         LP, Vista LP and Midland, and (b) from and after the Borrower Mergers,
         Prize LP.

                  "BRSI" means Fleet Boston Robertson Stephens Inc.,
         successor-in-interest to BankBoston Robertson Stephens Inc., a
         Massachusetts corporation which is an Affiliate of BankBoston.

                  "Loan Papers" means this Agreement, the First Amendment, the
         Notes, each Facility Guaranty now or hereafter executed (and any
         ratification thereof), the Mortgages, the Assignments and Amendments to
         Mortgages, each Parent Pledge Agreement now or hereafter executed, each
         Subsidiary Pledge Agreement now or hereafter executed, the Borrower
         Assumption Agreement, and all other certificates, documents or
         instruments delivered in connection with this Agreement, as the
         foregoing may be amended from time to time.

         1.2. Additional Definitions. Section 2.1 of the Credit Agreement shall
be amended to add the following definitions to such Section:

                  "Borrower Assumption Agreement" means an Assumption Agreement
         of even date with the First Amendment by and among Parent, Borrower,
         Administrative Agent and Banks, pursuant to which Borrower assumed and
         agreed to perform, as primary obligor, all obligations of Vista LP and
         Midland under the Credit Agreement and the other Loan Papers.

                  "Borrower Mergers" means, collectively, the mergers of Vista
         LP and Midland with and into Prize LP pursuant to, and in accordance
         with, the Borrower Merger Documents, with Prize LP being the surviving
         entity of each such merger.

                  "Borrower Merger Agreements" means, collectively: (a) that
         certain Plan of Merger dated on or about June 30, 2000, between Prize
         LP and Vista LP, relating to the merger of Vista LP with and into Prize
         LP, and (b) that certain Agreement of Merger dated on or about June 30,
         2000, between Prize LP and Midland, relating to the merger of Midland
         with and into Prize LP.


                                      -2-
<PAGE>   3

                  "Borrower Merger Certificates" means, collectively: (a) that
         certain Certificate of Merger dated on or about June 30, 2000, to be
         filed on or about June 30, 2000, with the Secretary of State of
         Delaware, relating to the merger of Vista LP with and into Prize LP,
         (b) those certain Articles of Merger dated on or about June 30, 2000,
         to be filed on or about June 30, 2000, with the Secretary of State of
         Texas, relating to the merger of Vista LP with and into Prize LP, (c)
         that certain Certificate of Merger dated on or about June 30, 2000, to
         be filed on or about June 30, 2000, with the Secretary of State of
         Delaware, relating to the merger of Midland with and into Prize LP, and
         (d) those certain Articles of Merger dated on or about June 30, 2000,
         to be filed on or about June 30, 2000, with the Secretary of State of
         Texas, relating to the merger of Midland with and into Prize LP, and
         certified copies of which shall subsequently be filed in such
         jurisdictions as Administrative Agent shall require.

                  "Borrower Merger Documents" means, collectively: (a) the
         Borrower Merger Agreements, (b) the Borrower Merger Certificates, and
         (c) all other material documents, instruments and agreements executed
         or delivered by any Credit Party pursuant to the Borrower Merger
         Agreements, the Borrower Merger Certificates or the Borrower Mergers.

                  "First Amendment" means that certain First Amendment to
         Amended and Restated Credit Agreement dated as of June 30, 2000,
         entered into by and among Parent, Prize LP, Administrative Agent and
         Banks a party thereto.

         1.3. Schedule 4. Schedule 4 to the Credit Agreement shall be replaced
in its entirety with the Schedule 4 attached hereto and made a part hereof.

         SECTION 2. (a) Restructuring Transactions. Parent and Borrower have
advised Banks that on or about June 30, 2000, Parent and Borrower intend to take
the following actions:

                           (i) Dissolution of Entities. Former Prize and Prize
                  Petroleum, L.L.C., an Oklahoma limited liability company
                  ("Prize LLC"), will be dissolved (collectively, the
                  "Dissolution") pursuant to, among other things, Certificates
                  of Dissolution to be filed with the Secretaries of State of
                  Delaware and Oklahoma, and authorizing resolutions and
                  consents adopted by the Board of Directors and stockholders or
                  other applicable and comparable authority of Former Prize and
                  Prize LLC approving the Dissolution;

                           (ii) Sunterra Distribution. Following the
                  Dissolution, the interest in Sunterra Petroleum, L.L.C., an
                  Oklahoma limited liability company ("Sunterra"), held by OGP
                  will be distributed (the "Sunterra Distribution") to Parent
                  pursuant to, among other things, an assignment to be executed
                  by OGP in favor of Parent, and authorizing resolutions adopted
                  by the Board of Directors of OGP approving the Sunterra
                  Distribution;

                           (iii) Partner Merger. Following the consummation of
                  the Dissolution and the Sunterra Distribution, (a) Vista LLC
                  will merge with and into PEC with PEC being the surviving
                  corporation (the "PEC Merger"), which PEC Merger will be


                                      -3-
<PAGE>   4

                  effectuated pursuant to, among other things, the filing of a
                  Certificate of Merger with the Secretary of State of Delaware,
                  and authorizing resolutions adopted by the Board of Directors
                  and stockholders or other applicable and comparable authority
                  of Vista LLC and PEC approving the PEC Merger, and (b) Vista
                  Resources I, Inc. ("Vista I") and Vista Resources, Inc.
                  ("Vista Resources"), each a Texas corporation, will merge with
                  and into Prize Operating Company, a Delaware corporation
                  ("POC"), with POC being the surviving corporation (the "POC
                  Merger," and together with the PEC Merger, the "Partner
                  Merger"), which POC Merger will be effectuated pursuant to,
                  among other things, the filing of a Certificate of Merger with
                  the Secretaries of State of Delaware and Texas, and
                  authorizing resolutions adopted by the Board of Directors and
                  stockholders or other applicable and comparable authority of
                  Vista I, Vista Resources and POC; and

                           (iv) Borrower Merger. Contemporaneously with the
                  consummation of the Partner Merger, Vista LP, Midland and
                  Sunterra will each merge with and into Borrower with Borrower
                  being the surviving entity (the "Borrower Merger"), which
                  Borrower Merger will be effectuated pursuant to, among other
                  things, the filing of Articles or Certificates of Merger, as
                  appropriate, with the Secretaries of State of Delaware,
                  Oklahoma and Texas, and authorizing resolutions and consents
                  (as applicable) adopted by the Board of Directors and
                  stockholders or other applicable and comparable authority and
                  (as applicable) the partners of Vista LP, Midland and Borrower
                  approving the Borrower Merger. Pursuant to, and after giving
                  effect to, the Borrower Merger, Borrower will assume and be
                  liable for all of Vista LP's and Midland's obligations under
                  the Credit Agreement, the Notes, the Mortgages (as applicable)
                  and each of the other Loan Papers to which either Vista LP or
                  Midland is a party.

         The Dissolution, the Sunterra Distribution, the Partner Merger and the
Borrower Merger are collectively referred to herein as the "Restructuring
Transactions").

         (b) Restrictions in Credit Agreement; Request for Consent and Waiver.
The Restructuring Transactions (or certain aspects thereof) are prohibited
pursuant to certain provisions of the Credit Agreement. Parent and Borrower have
requested that Banks consent to the Restructuring Transactions and waive any
provision of the Credit Agreement and the other Loan Papers to the extent that
any of the Restructuring Transactions violates any such provision or results in
a Default or Event of Default under the Credit Agreement or the other Loan
Papers.

         (c) Consent and Waiver. Subject to and upon the terms and conditions
set forth herein, and in reliance on the representations and warranties set
forth herein, Banks consent to the consummation by Parent and Borrower of the
Restructuring Transactions and waive compliance by Parent, Borrower and Parent's
Subsidiaries with each provision of the Credit Agreement and the other Loan
Papers, including, without limitation, Sections 9.3, 10.2, 10.4, 10.5, 10.6,
10.9 and 10.16 of the Credit Agreement to the extent, but only to the extent,
that the Restructuring Transactions, or any of them, violate such provisions or
result in a Default or Event of Default under the Credit Agreement or the other
Loan Papers.


                                      -4-
<PAGE>   5

         The consents and waivers herein contained are expressly limited as
follows: (a) such consents and waivers are limited solely to the consummation by
Parent, Borrower and the other Credit Parties of the Restructuring Transactions,
(b) such consents and waivers shall not be applicable to any provisions of any
Loan Paper other than provisions applicable to the Restructuring Transactions,
and (c) such consents and waivers are limited, one-time consents and waivers,
and nothing contained herein shall obligate Banks to grant any additional or
future consent or waiver of, or with respect to, any provision of any Loan
Paper.

         SECTION 3. Conditions Precedent. The effectiveness of the amendments to
the Credit Agreement contained in Section 1 hereof, and the consents and waivers
contained in Section 2 hereof, shall be effective only upon, and are conditioned
upon, the satisfaction of each of the following conditions precedent:

         3.1. Closing Deliveries. Administrative Agent shall have received each
of the following documents, instruments and agreements, each of which shall be
in form and substance and executed in such counterparts as shall be acceptable
to Administrative Agent and each Bank and each of which shall, unless otherwise
indicated, be dated on or about the Effective Date:

                  (a) copies of certificates of dissolution, certificates of
merger, merger agreements, assignments, conveyance documents and such other
documents, instruments and agreements executed in connection with the
Restructuring Transactions, and/or issued by the Secretaries of State of
Delaware, Oklahoma and Texas, evidencing the Restructuring Transactions,
together with copies of resolutions or comparable authorizations approving such
Restructuring Transactions duly adopted by the Board of Directors or other
comparable authority of each Credit Party a party thereto, or for which such
authorization is required (as applicable), accompanied by certificates of the
Secretary or comparable officer of Parent that such copies are true and correct
copies of resolutions duly adopted at a meeting of or (if permitted by
applicable Law and, if required by such Law, by the bylaws of Parent or such
other Credit Party) by the unanimous written consent of the Board of Directors
or other comparable authority of such Credit Party, and that such resolutions
constitute all the resolutions adopted with respect to such transactions, have
not been amended, modified or revoked in any respect, and are in full force and
effect as of the Effective Date;

                  (b) an Assumption Agreement duly executed by and among Parent,
Borrower, Administrative Agent and Banks, pursuant to which Borrower assumes and
agrees to perform, as primary obligor, all obligations of Vista LP and Midland
under the Credit Agreement and the other Loan Papers;

                  (c) a Parent Pledge Agreement duly executed and delivered by
Parent together with (i) certificates (or other evidence acceptable to
Administrative Agent) evidencing one hundred percent (100%) of the issued and
outstanding Equity of each direct Subsidiary of Parent of every class (after
giving effect to the Borrower Merger and the other Restructuring Transactions),
which certificates shall be duly endorsed or accompanied by stock powers
executed in blank (as applicable), and (ii) such financing statements executed
by Parent as Administrative Agent shall request to evidence and perfect the
Liens granted pursuant to such Parent Pledge Agreement;


                                      -5-
<PAGE>   6

                  (d) Subsidiary Pledge Agreements duly executed and delivered
by each Subsidiary of Parent (excluding Borrower) together with (i) certificates
(or other evidence acceptable to Administrative Agent) evidencing one hundred
percent (100%) of the issued and outstanding Equity of each Indirect Subsidiary
of Parent of every class (after giving effect to the Borrower Merger and the
other Restructuring Transactions), which certificates shall be duly endorsed or
accompanied by stock powers executed in blank (as applicable), and (ii) such
financing statements executed by each Subsidiary of Parent (other than Borrower
and after giving effect to the Borrower Merger and the other Restructuring
Transactions) as Administrative Agent shall request to evidence and perfect the
Liens granted pursuant to each such Subsidiary Pledge Agreement;

                  (e) Facility Guaranties (or reaffirmations of existing
Facility Guaranties) duly executed and delivered by Parent and each Subsidiary
of Parent (other than Borrower and PEC) after giving effect to the Borrower
Merger and the other Restructuring Transactions;

                  (f) a Certificate of Ownership Interests substantially in the
form of Exhibit B to the Credit Agreement, duly executed and delivered by an
Authorized Officer of Borrower;

                  (g) an opinion of Conner & Winters, A Professional
Corporation, special counsel to Parent and Borrower, favorably opining as to
such maters as Administrative Agent or Required Banks may request relative to
the Restructuring Transactions;

                  (h) a copy of the articles or certificate of incorporation,
certificate of limited partnership, or comparable charter documents, and all
amendments thereto, of each Credit Party (after giving effect to the
Restructuring Transactions), accompanied by a certificate that such copy is
true, correct and complete, issued by the appropriate Governmental Authority of
the jurisdiction of incorporation or organization of each such Credit Party, and
accompanied by a certificate of the Secretary or comparable Authorized Officer
of each such Credit Party, that such copy is true, correct and complete on the
Effective Date or, alternatively, a certificate of the Secretary or comparable
Authorized Officer of each such Credit Party that such documents have not been
amended or modified since February 9, 2000, and the same remain in full force
and effect;

                  (i) a copy of the bylaws, partnership agreement or comparable
documents, and all amendments thereto, of each Credit Party (after giving effect
to the Restructuring Transactions) accompanied by a certificate of the Secretary
or comparable Authorized Officer of each such Credit Party that such copy is
true, correct and complete as of the date hereof or, alternatively, a
certificate of the Secretary or comparable Authorized Officer of each such
Credit Party that such documents have not been amended or modified since
February 9, 2000, and the same remain in full force and effect;

                  (j) certain certificates and other documents issued by the
appropriate Governmental Authorities of such jurisdictions as Administrative
Agent has requested relating to the existence of each Credit Party (after giving
effect to the Restructuring Transactions) and to the effect that each Credit
Party is in good standing with respect to the payment of franchise and similar
Taxes and is duly qualified to transact business in such jurisdictions (after
giving effect to the Restructuring Transactions);


                                      -6-
<PAGE>   7

                  (k) copies of resolutions or comparable authorizations
approving this First Amendment, the Loan Papers executed in connection herewith
and authorizing the Restructuring Transactions and the transactions contemplated
by this First Amendment and the other Loan Papers executed in connection
herewith, duly adopted by the Board of Directors, partners or comparable
authority of each Credit Party a party to any Loan Paper, accompanied by
certificates of the Secretary or comparable officer or partner of each such
Credit Party (as applicable) that such copies are true and correct copies of
resolutions duly adopted at a meeting of or (if permitted by applicable Law and,
if required by such Law, by the Bylaws, or other charter documents of each such
Credit Party, as applicable) by the unanimous written consent of the Board of
Directors of each such Credit Party, as applicable, and that such resolutions
constitute all the resolutions adopted with respect to such transactions, have
not been amended, modified, or revoked in any respect, and are in full force and
effect as of the date hereof;

                  (l) to the extent not already delivered, a copy of each of the
Borrower Merger Documents accompanied by a certificate of an Authorized Officer
of Parent certifying that such copies are accurate and complete and represent
the complete understanding and agreement of the parties with respect to the
subject matter thereof; and

                  (m) such other documents, instruments and agreement as
Administrative Agent or any Bank may request in connection with the
Restructuring Transactions and/or this First Amendment.

         3.2. Restructuring Transactions. The Borrower Merger and the other
Restructuring Transactions shall have occurred pursuant to the terms of the
Borrower Merger Documents and the other applicable agreements and all
certificates of dissolution, certificates of merger and other applicable
documents shall have been filed with the appropriate Governmental Authorities of
the applicable jurisdictions.

         3.3. No Material Adverse Change. No Material Adverse Change (other than
in connection with the Restructuring Transactions) shall have occurred since
February 9, 2000.

         3.4. No Legal Prohibition. The transactions contemplated by this First
Amendment and the Restructuring Transactions shall be permitted by applicable
Law and regulation and shall not subject Administrative Agent, any Bank, Parent,
Borrower or any Credit Party to any Material Adverse Change.

         3.5. No Litigation. No litigation, arbitration or similar proceeding
shall be pending or threatened which calls into question the validity or
enforceability of the Restructuring Transactions, this First Amendment, the
other Loan Papers or the transactions contemplated hereby or thereby.

         3.6. Legal Expenses. All reasonable fees and expenses of counsel to
Administrative Agent incurred by Administrative Agent in connection with the
preparation, negotiation and execution of this First Amendment and all related
documents shall have been paid.


                                      -7-
<PAGE>   8

         3.7. Other Matters. All matters related to the Restructuring
Transactions, this First Amendment, the other Loan Papers, Parent, Borrower and
the other Credit Parties shall be acceptable to each Bank in its sole
discretion, and Parent and Borrower shall have delivered to Administrative Agent
and each Bank such evidence as they shall request to substantiate any matters
related to the Restructuring Transactions, this First Amendment and the other
Loan Papers, as Administrative Agent or any Bank shall request.

         SECTION 4. Representations and Warranties. In order to induce
Administrative Agent and Banks to enter into this First Amendment, Parent and
Borrower hereby jointly and severally represent and warrant to Administrative
Agent and each Bank that:

         4.1. Accuracy of Representations and Warranties. Each representation
and warranty of the Credit Parties contained in the Loan Papers, modified as
necessary for the effect of the Restructuring Transactions, is true and correct
in all material respects as of the date hereof and after giving effect to the
Restructuring Transactions (except to the extent that such representations and
warranties are expressly made as of a particular date, in which event such
representations and warranties were true and correct as of such date);

         4.2. Absence of Defaults. After giving effect hereto, neither a Default
nor an Event of Default has occurred which is continuing;

         4.3. No Defense. No Credit Party has any defenses to payment,
counterclaims or rights of set-off with respect to the Obligations on the date
hereof;

         4.4. Organizational Structure; Nature of Business. PEC exists for the
sole purpose of owning a limited partner interest in Borrower. Each Credit Party
(other than PEC) is engaged only in the business of acquiring, exploring,
developing and operating Mineral Interests and the production, marketing,
processing and transporting of Hydrocarbons therefrom. Schedule 4 attached
hereto accurately reflects, as of the date hereof, and after giving effect to
the Restructuring Transactions: (i) the jurisdiction of incorporation or
organization of each Credit Party, (ii) each jurisdiction in which each Credit
Party is qualified to transact business as a foreign corporation or foreign
partnership, (iii) the authorized, issued and outstanding stock or partnership
interests of each Credit Party, including the names of (and number of shares or
other equity interests held by) the record and beneficial owners of such
interests, and (iv) all outstanding warrants, options, subscription rights,
convertible securities or other rights to purchase capital stock or partnership
interests of each Credit Party. Except as set forth in this Section 4.4 and in
Schedule 4 hereto, no Person holds record or beneficial ownership of any capital
stock or other equity interest in Borrower or any other Subsidiary of Parent or
any other right or option to acquire any capital stock or other equity interest
in Borrower or any other Subsidiary of Parent and, without limiting the
foregoing, there are not outstanding any warrants, options, subscription rights
or other rights to purchase stock or other equity interests in Borrower or any
other Subsidiary of Parent. No Credit Party has made or presently holds any
Investments other than Permitted Investments. Except as set forth in Schedule 4
hereto, Borrower does not have any Subsidiaries, and no Credit Party is a
partner or joint venturer in any partnership or joint venture or a member of any
unincorporated association.


                                      -8-
<PAGE>   9

         4.5. Merger Documents. Borrower has provided each Bank with a true and
correct copy of each of the Borrower Merger Documents, including all amendments
and modifications thereto.

SECTION 5. Miscellaneous.

5.1. Reaffirmation of Loan Papers; Extension of Liens. Any and all of the terms
and provisions of the Credit Agreement and the Loan Papers shall, except as
amended and modified hereby, remain in full force and effect. Parent and
Borrower hereby extend the Liens securing the Obligations until the Obligations
have been paid in full, and agree that the amendments and modifications herein
contained shall in no manner affect or impair the Obligations or the Liens
securing payment and performance thereof.

5.2. Parties in Interest. All of the terms and provisions of this First
Amendment shall bind and inure to the benefit of the parties hereto and their
respective successors and assigns.

5.3. Counterparts. This First Amendment may be executed in counterparts, and all
parties need not execute the same counterpart; however, no party shall be bound
by this First Amendment until this First Amendment has been executed by Parent,
Borrower, Administrative Agent and Banks at which time this First Amendment
shall be binding on, enforceable against and inure to the benefit of Parent,
Borrower, Administrative Agent and each Bank. Facsimiles shall be effective as
originals.

5.4. Legal Expenses. Parent and Borrower hereby jointly and severally agree to
pay on demand all reasonable fees and expenses of counsel to Administrative
Agent incurred by Administrative Agent in connection with the preparation,
negotiation and execution of this First Amendment and all related documents.

5.5. COMPLETE AGREEMENT. THIS FIRST AMENDMENT, THE CREDIT AGREEMENT AND THE
OTHER LOAN PAPERS REPRESENT THE FINAL AGREEMENT AMONG THE PARTIES AND MAY NOT BE
CONTRADICTED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR ORAL AGREEMENTS OF THE
PARTIES. THERE ARE NO UNWRITTEN ORAL AGREEMENTS BETWEEN THE PARTIES.

5.6. Headings. The headings, captions and arrangements used in this First
Amendment are, unless specified otherwise, for convenience only and shall not be
deemed to limit, amplify or modify the terms of this First Amendment, nor affect
the meaning thereof.

5.7 Effectiveness. This First Amendment shall be effective only upon (a) the
execution hereof by Banks, and (b) the satisfaction of each of the conditions
precedent set forth in Section 3 hereof.

IN WITNESS WHEREOF, the parties hereto have caused this First Amendment to be
duly executed by their respective Authorized Officers on the date and year first
above written.


                           [Signature pages to follow]


                                      -9-
<PAGE>   10

                                 SIGNATURE PAGE
                                       TO
                     FIRST AMENDMENT TO AMENDED AND RESTATED
                          CREDIT AGREEMENT BY AND AMONG
                PRIZE ENERGY CORP., PRIZE ENERGY RESOURCES, L.P.,
                  FLEET NATIONAL BANK, AS ADMINISTRATIVE AGENT,
                          AND THE BANKS A PARTY THERETO


PARENT:

PRIZE ENERGY CORP.,
a Delaware corporation


By: /s/ Lon C. Kile
    ----------------------------------
        Lon C. Kile,
        President


BORROWER:

PRIZE ENERGY RESOURCES, L.P.,
a Delaware limited partnership

By: Prize Operating Company,
    a Delaware corporation,
    its sole general partner


By: /s/ Lon C. Kile
    ----------------------------------
    Lon C. Kile,
    President




<PAGE>   11

                                 SIGNATURE PAGE
                                       TO
                     FIRST AMENDMENT TO AMENDED AND RESTATED
                          CREDIT AGREEMENT BY AND AMONG
                PRIZE ENERGY CORP., PRIZE ENERGY RESOURCES, L.P.,
                  FLEET NATIONAL BANK, AS ADMINISTRATIVE AGENT,
                          AND THE BANKS A PARTY THERETO


ADMINISTRATIVE AGENT:

FLEET NATIONAL BANK,
successor-in-interest to
BankBoston, N.A., as
Administrative Agent



By: /s/ Timothy J. Norton
    ---------------------------------
Name:    Timothy J. Norton
      -------------------------------
Title:   Director
       ------------------------------


FLEET NATIONAL BANK,
successor-in-interest to
BankBoston, N.A., as a Bank



By: /s/ Timothy J. Norton
    ---------------------------------
Name:    Timothy J. Norton
      -------------------------------
Title:   Director
       ------------------------------



<PAGE>   12

                                 SIGNATURE PAGE
                                       TO
                     FIRST AMENDMENT TO AMENDED AND RESTATED
                          CREDIT AGREEMENT BY AND AMONG
                PRIZE ENERGY CORP., PRIZE ENERGY RESOURCES, L.P.,
                  FLEET NATIONAL BANK, AS ADMINISTRATIVE AGENT,
                          AND THE BANKS A PARTY THERETO



FIRST UNION NATIONAL BANK



By:  /s/ Paul N. Riddle
    ---------------------------------
Name:    Paul N. Riddle
      -------------------------------
Title:   Senior Vice President
       ------------------------------


<PAGE>   13

                                 SIGNATURE PAGE
                                       TO
                     FIRST AMENDMENT TO AMENDED AND RESTATED
                          CREDIT AGREEMENT BY AND AMONG
                PRIZE ENERGY CORP., PRIZE ENERGY RESOURCES, L.P.,
                  FLEET NATIONAL BANK, AS ADMINISTRATIVE AGENT,
                          AND THE BANKS A PARTY THERETO



CIBC INC.



By:  /s/ M. Beth Miller
    ---------------------------------
Name:    M. Beth Miller
      -------------------------------
Title:   Authorized Signatory
       ------------------------------


<PAGE>   14

                                 SIGNATURE PAGE
                                       TO
                     FIRST AMENDMENT TO AMENDED AND RESTATED
                          CREDIT AGREEMENT BY AND AMONG
                PRIZE ENERGY CORP., PRIZE ENERGY RESOURCES, L.P.,
                  FLEET NATIONAL BANK, AS ADMINISTRATIVE AGENT,
                          AND THE BANKS A PARTY THERETO



BANK ONE, TEXAS, N.A.



By:  /s/ Mynan C. Feldman
    --------------------------------
Name:    Mynan C. Feldman
      ------------------------------
Title:   First Vice President
       -----------------------------


<PAGE>   15

                                 SIGNATURE PAGE
                                       TO
                     FIRST AMENDMENT TO AMENDED AND RESTATED
                          CREDIT AGREEMENT BY AND AMONG
                PRIZE ENERGY CORP., PRIZE ENERGY RESOURCES, L.P.,
                  FLEET NATIONAL BANK, AS ADMINISTRATIVE AGENT,
                          AND THE BANKS A PARTY THERETO



CHRISTIANIA BANK



By:  /s/ Peter M. Dodge
    ---------------------------------
Name:    Peter M. Dodge
    ---------------------------------
Title:   Senior Vice President
       ------------------------------



By:  /s/ William S. Phillips
    ---------------------------------
Name:    William S. Phillips
    ---------------------------------
Title:   First Vice President
       ------------------------------


<PAGE>   16

                                 SIGNATURE PAGE
                                       TO
                     FIRST AMENDMENT TO AMENDED AND RESTATED
                          CREDIT AGREEMENT BY AND AMONG
                PRIZE ENERGY CORP., PRIZE ENERGY RESOURCES, L.P.,
                  FLEET NATIONAL BANK, AS ADMINISTRATIVE AGENT,
                          AND THE BANKS A PARTY THERETO




BANK OF SCOTLAND



By:  /s/  Joseph Fratus
    -------------------------------
Name:    Joseph Fratus
      -----------------------------
Title:   Vice President
       ----------------------------


<PAGE>   17

                                 SIGNATURE PAGE
                                       TO
                     FIRST AMENDMENT TO AMENDED AND RESTATED
                          CREDIT AGREEMENT BY AND AMONG
                PRIZE ENERGY CORP., PRIZE ENERGY RESOURCES, L.P.,
                  FLEET NATIONAL BANK, AS ADMINISTRATIVE AGENT,
                          AND THE BANKS A PARTY THERETO



COMERICA BANK - TEXAS



By:  /s/ Jeffrey H. Rathkamp
    ----------------------------------
Name:    Jeffrey H. Rathkamp
      --------------------------------
Title:   Vice President
       -------------------------------


<PAGE>   18

                                 SIGNATURE PAGE
                                       TO
                     FIRST AMENDMENT TO AMENDED AND RESTATED
                          CREDIT AGREEMENT BY AND AMONG
                PRIZE ENERGY CORP., PRIZE ENERGY RESOURCES, L.P.,
                  FLEET NATIONAL BANK, AS ADMINISTRATIVE AGENT,
                          AND THE BANKS A PARTY THERETO



FORTIS CAPITAL CORP.



By:  /s/ Deirdre Sanborn
    ---------------------------------
Name:    Deirdre Sanborn
      -------------------------------
Title:   Vice President
       ------------------------------


By:  /s/ Darrell W. Holley
    ---------------------------------
Name:    Darrell W. Holley
      -------------------------------
Title:   Managing Director
       ------------------------------


<PAGE>   19

                                 SIGNATURE PAGE
                                       TO
                     FIRST AMENDMENT TO AMENDED AND RESTATED
                          CREDIT AGREEMENT BY AND AMONG
                PRIZE ENERGY CORP., PRIZE ENERGY RESOURCES, L.P.,
                  FLEET NATIONAL BANK, AS ADMINISTRATIVE AGENT,
                          AND THE BANKS A PARTY THERETO



THE BANK OF NOVA SCOTIA



By:  /s/ Illegible
    ---------------------------------
Name:    Illegible
      -------------------------------
Title:   Manager - Loan Operations
       ------------------------------


<PAGE>   20

                                 SIGNATURE PAGE
                                       TO
                     FIRST AMENDMENT TO AMENDED AND RESTATED
                          CREDIT AGREEMENT BY AND AMONG
                PRIZE ENERGY CORP., PRIZE ENERGY RESOURCES, L.P.,
                  FLEET NATIONAL BANK, AS ADMINISTRATIVE AGENT,
                          AND THE BANKS A PARTY THERETO



THE FROST NATIONAL BANK



By:  /s/  John S. Warren
    -------------------------------
Name:    John S. Warren
      -----------------------------
Title:   Senior Vice President
       ----------------------------


<PAGE>   21


                                   SCHEDULE 4

                            ORGANIZATIONAL STRUCTURE



PRIZE ENERGY CORP. (FORMERLY KNOWN AS VISTA ENERGY RESOURCES, INC.) ("PARENT")

(i)      Delaware corporation

(ii)     Texas

(iii)    Authorized:
         50,000,000 shares of common stock, par value $.01 per share
         5,000,000 shares of series A 6% convertible preferred stock, par value
         $.01 per share 5,000,000 shares of undesignated preferred stock, par
         value $.01 per share

         Issued:
         13,266,929 shares of common stock (as of April 18, 2000)

(iv)     Warrants to purchase 2,211,300 shares of common stock (as of April 1,
         2000) Options to purchase 11,892,501 shares of common stock (as of
         April 1, 2000)



PRIZE OPERATING COMPANY ("POC")

         (i)      Delaware corporation

         (ii)     Arkansas          Kansas
                  Louisiana         Mississippi
                  New Mexico        North Dakota
                  Oklahoma          Texas
                  Utah              Wyoming

         (iii)    Authorized:  1,000 shares of common stock

                  Issued:  1,000 shares of common stock (100% owned by Parent)

         (iv)     None



<PAGE>   22



PEC (DELAWARE), INC. ("PEC")

         (i)      Delaware corporation

         (ii)     None

         (iii)    Authorized:  1,000 shares of common stock

                  Issued:  1,000 shares of common stock (100% owned by Parent)

         (iv)     None


OKLAHOMA GAS PROCESSING, INC. ("OGP")

         (i)      Delaware corporation

         (ii)     Oklahoma
                  Texas

         (iii)    Authorized:  1,000 shares of common stock

                  Issued:  1,000 shares of common stock (100% owned by Parent)

         (iv)     None


PRIZE ENERGY RESOURCES, L.P.

         (i)      Delaware limited partnership

         (ii)     Kansas            Louisiana
                  Mississippi       New Mexico
                  North Dakota      Oklahoma
                  Texas             Utah
                  Wyoming

         (iii)    General Partner (1%):  POC
                  Limited Partner (99%):  PEC

         (iv)     None


                                       2