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                                                                     EXHIBIT 4.2


                       FIRST AMENDMENT TO RIGHTS AGREEMENT

         FIRST AMENDMENT TO RIGHTS AGREEMENT, dated as of September 30, 1997,
between VMARK Software, Inc., a Delaware corporation (the "Company"), and State
Street Bank and Trust Company (the "Rights Agent"). Capitalized terms not
defined herein shall have the meanings assigned to such terms within the Rights
Agreement (as defined below).

                                    RECITALS

         A.   The Company and the Rights Agent entered into a Rights Agreement
dated as of June 12, 1996 (the "Rights Agreement") relating to the declaration
of a dividend of one Preferred Share purchase right (a "Right") for each Common
Share of the Company outstanding as of the Close of Business on June 12, 1996
and for each Common Share of the Company that shall become outstanding between
the Record Date and the earlier of the Distribution Date and the Expiration
Date, and in certain circumstances after the Distribution Date.

         B.   The Rights Agreement sets forth, among other things, the terms and
conditions pursuant to which the holders of Rights may exercise such Rights.

         C.   The Company intends to enter into a Merger Agreement dated in
October, 1997 (the "Merger Agreement") with Unidata, Inc., a Colorado
corporation ("Unidata"), pursuant to which Unidata will merge with and into the
Company.

         D.   The Company and the Rights Agent have determined that the entering
into of the Merger Agreement by the Company and the transactions contemplated
thereby should not affect either party's rights under the Rights Agreement.

         E.   Pursuant to Section 27 of the Rights Agreement, the Company has
provided the Rights Agent with a certificate from an appropriate officer stating
that this Amendment is in compliance with such Section 27.

         F.   In connection with the transactions pursuant to the Merger
Agreement, the Company and the Rights Agent have agreed to amend the Rights
Agreement as set forth herein.

         For good and valuable consideration, the receipt and sufficiency of
which are hereby acknowledged, the parties hereto agree to amend the Rights
Agreement as follows:

         1.   The definition of "Acquiring Person" appearing in Section 1(a) of
              the Rights Agreement is hereby amended by inserting the phrase "or
              an Exempt Person" immediately following the phrase "or an entity
              holding Common Shares for or pursuant to the terms of any such
              plan" in the first sentence of such definition.


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         2.   Section 1 of the Rights Agreement is hereby amended by inserting
              the following definition immediately following the definition of
              "Equivalent Shares" appearing in such Section:

              "Exempt Person" shall mean, collectively and each separately,
              James T. Dresher, Glenangus Holdings Corporation (so long as one
              or more Exempt Persons jointly own at least 50% or more of the
              voting securities of such corporation), Jeffrey M. Dresher, James
              T. Dresher, Jr., the Joshua M. Dresher Irrevocable Trust, the
              Marcie A. Dresher Irrevocable Trust, and the James T. Dresher, III
              Irrevocable Trust. Notwithstanding the foregoing sentence, a
              Person who would otherwise be an Exempt Person shall immediately
              lose its status as an Exempt Person upon the occurrence of any of
              the following: (i) such Person who or which, together with all
              Affiliates and Associates of such Person, shall be the Beneficial
              Owner of 40% or more of the Common Shares of the Company then
              outstanding; (ii) such Person shall make a public announcement
              that such Person, directly or indirectly, individually or
              collectively with a group, intends to commence a tender or
              exchange offer for any Common Shares of the Company then
              outstanding if, assuming the successful consummation thereof, such
              Person would be the Beneficial Owner of 30% or more of the shares
              of Common Stock then outstanding; (iii) such Person shall enter
              into any voting agreement or voting trust pursuant to which such
              Person agrees to vote his shares with or for any other holder of
              Common Shares of the Company (other than any voting agreement or
              voting trust that is with other Exempt Persons); and (iv) such
              Person shall grant a proxy or power of attorney to any other
              Person (other than an officer, director, agent or employee of the
              Company and other than to another Exempt Person) for the purpose
              of voting such Person's Common Shares. The assignee, transferee or
              successor to any Exempt Person shall not constitute an Exempt
              Person (unless such assignee, transferee or successor was an
              Exempt Person immediately prior to such assignment, transfer or
              successor) without the prior written consent of the Company, which
              consent may be withheld in the Company's absolute discretion.
              Notwithstanding the foregoing, no Person shall be deemed to be an
              Acquiring Person either (i) as the result of an acquisition of
              Common Shares by the Company which, by reducing the number of
              shares outstanding, increases the proportionate number of shares
              beneficially owned by such Exempt Person to 40% or more of the
              Common Shares of the Company then outstanding; PROVIDED, HOWEVER,
              that if an Exempt Person shall become the Beneficial Owner of 40%
              or more of the Common Shares of the Company then outstanding by
              reason of share purchases by the Company and shall, after such
              share purchases by the company, become the Beneficial Owner of any
              additional Common Shares of the Company, then such Exempt Person
              shall be deemed to be an Acquiring Person, or (ii) if within eight
              days after such Exempt Person would otherwise become an Acquiring
              Person (but for the operation of this clause (ii)), such Person
              notifies the Board of Directors that such Exempt Person did so
              inadvertently and within 

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              two days after such notification such Exempt Person is the 
              Beneficial Owner of less than 40% of the outstanding Common 
              Shares.

         3.   The Definition of "Continuing Director" appearing in section 1(g)
              of the Rights Agreement is hereby amended by inserting ", or (iii)
              David Brunel, James T. Dresher, or John F. Schaefer" immediately
              following the phrase "approved by a majority of the Continuing
              Directors" in such definition.

         4.   Except to the extent specifically amended hereby, all terms of the
              Rights Agreement shall remain in full force and effect.

         5.   This Amendment may be executed in any number of counterparts, all
              of which taken together shall constitute one and the same
              instrument.

         6.   This Agreement shall be deemed to be a contract made under the
              laws of the State of Delaware and for all purposes shall be
              governed by and construed in accordance with the laws of such
              State applicable to contracts to be made and performed entirely
              within such State.

         IN WITNESS WHEREOF, the parties hereto have duly executed and delivered
this Agreement as of the date first above written.

                                            VMARK SOFTWARE, INC.


                                            By /s/ Richard N. Hoehn
                                               ----------------------------
                                              Name: Richard N. Hoehn
                                              Title: Secretary

                                            STATE STREET BANK AND TRUST
                                            COMPANY


                                            By /s/ Margaret Prentice
                                               ----------------------------
                                              Name: Margaret Prentice
                                              Title: Administration Manager



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