<PAGE>

                                                                   Exhibit 10.20

                                   SUBLEASE


     THIS SUBLEASE (this "Sublease" or this "Agreement") is made as of June
26, 2000, between APTIS, INC., d/b/a APTIS SOFTWARE, whose address is 7411
John Smith, Suite 200, San Antonio, Texas 78229-4898, hereinafter called
"Sublessor," VIGNETTE CORPORATION, whose address is 901 South MoPac, Building
III, Austin, Texas 78746, hereinafter called "Sublessee," and BILLING CONCEPTS
CORP., whose address is 7411 John Smith, Suite 200, San Antonio, Texas 78229-
4898 ("Guarantor").

                                  WITNESSETH:

     WHEREAS, PRENTISS PROPERTIES ACQUISITION PARTNERS, LP (the "Prime Lessor")
and Sublessor executed that certain Lease Agreement dated 11th day of November
1999, herein called the "Prime Lease," for a certain premises in the building
known as Two Barton Skyway, 1601 South MoPac Expressway, Austin, Texas 78746
(the "Building"); and

     WHEREAS, Guarantor executed a certain "Guaranty" (herein so called), in
connection with the Prime Lease; and

     WHEREAS, Sublessee desires to sublease such premises from Sublessor on the
terms and conditions set forth herein.

     Terms used herein which are defined in the Prime Lease, and which are not
otherwise defined herein, shall be given the same meanings herein as are
ascribed to them in the Prime Lease.

     NOW, THEREFORE, for and in consideration of the foregoing and for other
good and valuable consideration and of the mutual agreements hereinafter set
forth, Sublessor and Sublessee stipulate, covenant and agree as follows:

Premises       1.   Sublessor does hereby agree to sublease to Sublessee, and
               Sublessee does hereby agree to sublease from Sublessor, a portion
               of the Building located on the fourth (4th) and second (2nd)
               floors of the Building (collectively referred to as the "Sublease
               Premises"). The Sublease Premises contains a total of 75,449
               rentable square feet ("RSF") of space and consists of the
               following:

               4th Floor - 50,738 RSF (the entire 4th Floor, C & D Wings)
               ----------------------

               2nd Floor - 24,711 RSF (the D Wing of the 2nd Floor)
               ----------------------

                                     Page 1
<PAGE>

Term           2.  The Term of this Sublease shall commence (each, a "Sublease
               Commencement Date") as follows:

               4th Floor      The "Commencement Date" as set forth in the Prime
               ---------
                              Lease.

               2nd Floor      The first anniversary of the "Commencement Date"
               ---------
                              as set forth in the Prime Lease

               The term of this of this Sublease shall continue until the last
               day of the calendar month containing the 5th anniversary of the
               4th Floor Sublease Commencement Date.

Uses           3.  Sublessee shall use and occupy the Sublease Premises for
               office purposes and for no other purpose.

Rent and
Additional     4.  (a) Sublessee shall pay Sublessor rent (the "Rent") as
Rent           follows:

                   4th Floor  $21.40 per RSF, per annum

                   2nd Floor  In the event Sublessee occupies the 2nd Floor
                              space prior to October 1, 2001, Sublessee shall
                              pay rent at the rate of $3.00 per RSF, per annum
                              until October 1, 2001, at which time the Sublessee
                              shall pay rent at the rate of $21.40 per RSF, per
                              annum.

               Rent shall be payable in advance on the first day of each month
               during the term of this Sublease without deduction, set off, or
               demand. As to each portion of the Sublease Premises, Rent shall
               commence on the applicable Sublease Commencement Date, except as
               set forth above. Rent for any portion of a month shall be
               prorated on a thirty (30) day basis. Rent payments will be
               delivered to Sublessor, or such other place as Sublessor may
               designate.

               (b) Sublessee shall pay as additional rent "Tenant's Share of
               Operating Costs" as defined in the Prime Lease, incurred through
               the end of the Term of this Sublease, which are provided for
               under Article 5 of the Prime Lease entitled "Operating Costs."
               Such Additional Rent will be payable by Sublessee to Sublessor at
               the same time and in the same manner as it is payable by
               Sublessor to Prime Lessor under the Prime Lease.

                                     Page 2
<PAGE>

               (c)  Sublessee shall pay the "Base Rent" and "Additional Rent"
               components of the Sublease Rent directly to Prime Lessor, and the
               balance of the Rent shall be paid to Sublessor hereunder.
               Pursuant to the Consent to Sublease, as hereinafter defined,
               Prime Lessor has agreed to accept payment of the "Base Rent" and
               "Additional Rent" components of the Sublease Rent, directly from
               Sublessee.

Preparation
for
Occupancy      5.   (a)  Sublessee shall accept the Sublease Premises in the
               "Tenant Finish Ready Condition," as provided for under Article 1
               and Attachment 2 to Exhibit C of the Prime Lease.

               (b)  Sublessor is not responsible for and shall not be required
               to perform work of any kind or nature with respect to the
               Sublease Premises. Construction of the Leasehold Improvements
               shall be the responsibility of Sublessee in accordance with
               Exhibit C of the Lease and the Consent to Sublease.

Security
Deposit        6.   Sublessee shall pay a security deposit (the "Security
               Deposit") as follows:

               (a)  1st Month's Rent - Cash or collected funds in the amount of
                    ----------------
                    $128,536.27, with the Base Rent component thereof to be paid
                    directly to Prime Lessor and the balance to be paid to
                    Sublessor upon the execution hereof, and applied to the
                    payment of the first month's Rent.

               (b)  Sublease Security Deposit - Cash or collected funds in the
                    -------------------------
                    amount of $1,850,000.00 to be paid directly to Prime Lessor
                    upon the execution hereof, which funds will serve as the
                    Security Deposit required by Article 21 of the Prime Lease,
                    and will be deposited and applied or returned to Sublessee
                    as provided in the Lease, and as modified by the Consent to
                    Sublease.  Sublessor shall have no responsibility for the
                    Security Deposit

               (c)  The Security Deposit shall be security for the performance
                    by Sublessee of its obligations under this Sublease.  The
                    Security Deposit is not an advance payment of Rent or a
                    measure of Sublessor's damages for a default by Sublessee.
                    If Sublessee defaults in the performance of any of its
                    obligations under this

                                     Page 3
<PAGE>

                    Sublease or the Prime Lease, Sublessor may, without
                    prejudice to any other remedy, instruct the Prime Lessor to
                    use all or a portion of the Security Deposit to the extent
                    necessary to make good any shortages in Rent or any other
                    sum for which Sublessee is in default and any other damage,
                    injury, expense, or liability caused to Sublessor by the
                    default, or any default under the Prime Lease. If any part
                    of the Security Deposit is applied, Sublessee shall pay to
                    Sublessor or deliver to Sublessor the amount necessary to
                    restore the Security Deposit to its original amount.

Renewal        7.   Sublessee shall have the right to renew this Sublease on the
               Sublease Premises for one additional five (5) year term, provided
               Sublessee shall provide Sublessor with written notice of its
               intent to renew no less than 180 days and no more than 360 days
               prior to the expiration of the Term of this Sublease.  All
               economic terms and conditions shall be the "Fair Market Rental
               Rate," as defined in Article 28 of the Prime Lease, determined by
               Sublessor and Sublessee in that manner.  In no event will the
               Rent of the extended term be less than the Rent of the original
               term.

Incorporation
of Prime
Lease          8.   (a)  A true and correct copy of the Prime Lease is attached
               as Exhibit "A."  Sublessor warrants and represents to Sublessee
               that the Prime Lease has not been amended, and no party is in
               default under the terms of the Prime Lease.

               (b)  Sublessor will not modify the Prime Lease in a manner which
               affects the use and occupancy of the Sublease Premises without
               the consent of Sublessee, which consent shall not be unreasonably
               withheld or delayed and shall be deemed given thirty (30) days
               after written request if no objection is made in writing
               specifically stating how such modification affects the use and
               occupancy of the Sublease Premises.

               (c)  This Sublease is subject to all of the terms of the Prime
               Lease with the same force and effect as if fully set forth herein
               at length, excepting only as otherwise specifically provided
               herein. Subject to Sublessee's performance of this Sublease,
               Sublessor covenants and agrees to pay to Prime Lessor all rents
               and additional rents due and payable to Prime Lessor under the
               Prime Lease. All of the terms with which Sublessor is bound to
               comply under the Prime Lease shall, to the extent only that they
               apply to the Sublease Premises and except as otherwise provided
               herein, be binding upon

                                     Page 4
<PAGE>

               Sublessee, and all of the obligations of Prime Lessor set forth
               in the Prime Lease shall, to the extent that they apply to the
               Sublease Premises, be binding upon Sublessor, and inure to
               Sublessee's benefit. It is the intention of the parties that,
               except as otherwise provided in this Sublease, the relationship
               between Sublessor and Sublessee shall be governed by the language
               of the various articles of the Prime Lease as if they were typed
               out in this Sublease in full, and the words "Landlord," "Tenant"
               and "Lease" as used in the Prime Lease, shall read, respectively,
               "Sublessor," "Sublessee" and "Sublease."

               (d)  For the purposes of this Sublease, the following provisions
               of the Prime Lease, and the reference to them in the body of the
               Prime Lease are not applicable, or are modified by the terms of
               this Sublease:

                    - Article 1, Basic Lease Information and Certain
                      Definitions:

                         -  subparagraph F, entitled "Commencement Date" to the
                            extent in conflict with this Sublease is modified by
                            this Sublease.

                         -  subparagraph G, entitled "Expiration Date" to the
                            extent in conflict with this Sublease is modified by
                            this Sublease.

                         -  subparagraph L, entitled "Rent" to the extent in
                            conflict with this Sublease is modified by this
                            Sublease.

                         - subparagraph Q, entitled "Basic Rent" to the extent
                           in conflict with this Sublease is modified by this
                           Sublease.

                         -   Article 28, entitled "Renewal Options" is deleted;
                         provided, however, that the procedure set forth therein
                         to determine "Fair Market Rental Rate" will still be
                         employed should Sublessee exercise its right to renew
                         this Sublease in accordance with Section 7 hereof.

Quiet
Enjoyment      9.   (a)  Sublessor covenants and agrees with Sublessee that upon
               Sublessee paying the Rent and additional rent reserved in this
               Sublease and observing and performing all of the other
               obligations, terms, covenants and conditions of this Sublease on
               Sublessee's part to be observed and performed, Sublessee may
               peaceably and quietly enjoy the Sublease

                                     Page 5
<PAGE>

               Premises (and all rights and appurtenances thereto provided in
               the Prime Lease) during the term; provided, however, that, except
               as hereafter expressly provided to the contrary, this Sublease
               shall automatically terminate upon termination of the Prime
               Lease. Sublessee shall have no claim against Sublessor unless
               such termination was caused by the default of Sublessor in the
               performance of its obligations under the Prime Lease which have
               been assumed by Sublessor under this Sublease and have not been
               assumed by Sublessee hereunder; provided, however, if the Prime
               Lease is terminated as the result of a default by Prime Lessor,
               then Sublessee shall have the right to bring a claim against
               Prime Lessor and Sublessor hereby appoints Sublessee as its
               attorney in fact under the Prime Lease for the purpose of
               bringing such a claim, but only to the extent such claim relates
               to the Sublease Premises, and Sublessor shall cooperate with
               Sublessee in any such claim.

               (b)  Sublessee covenants and agrees that Sublessee shall not do
               or suffer or permit anything to be done which would constitute a
               default under the Prime Lease or would cause the Prime Lease to
               be canceled, terminated or forfeited by virtue of any rights of
               cancellation, termination, or forfeiture reserved or vested in
               Prime Lessor under the Prime Lease, and that Sublessee will
               indemnify and hold harmless Sublessor from and defend Sublessor
               against all claims, liabilities, losses and damages of any kind
               whatsoever (excepting special and consequential damages) that
               Sublessor may incur by reason of, resulting from or arising out
               of any such cancellation, termination or forfeiture.

Notices        10.  Any notice, demand or request under this Sublease shall be
               in writing and shall be considered property delivered when
               addressed as herein provided and delivered by registered or
               certified mail (return receipt requested) which is deposited in
               the United States general or branch post office, or delivered by
               private express mail service. Any notice, demand or request by
               Sublessor to Sublessee shall be addressed to Sublessee and
               Sublessor at the addresses above, until either shall direct
               otherwise in writing.

               Rejection or other refusal to accept or the inability to deliver
               because of a changed address of which no notice was given shall
               be deemed to be receipt of the notice, demand or request sent.

Assignment

                                     Page 6
<PAGE>

and
Subletting     11.  (a)  Sublessee may not, without the prior written consent of
               Sublessor and Prime Lessor (which consent shall not be
               unreasonably conditioned, withheld, or delayed): (1) assign or
               transfer this Sublease or any interest therein; (2) permit any
               assignment of this Sublease or any interest therein by operation
               of law; (3) sublet the Subleased Premises or any part thereof;
               (4) grant any license, concession, or other right of occupancy of
               any portion of the Subleased Premises; or (5) mortgage, pledge,
               or otherwise encumber its interest in this Sublease. Sublessor's
               consent to any assignment or subletting is not a waiver of
               Sublessor's right to approve or disapprove any subsequent
               assignment or subletting. Sublessee shall remain jointly and
               severally liable for the payment of Rent and performance of all
               other obligations under this Sublease after any assignment or
               subletting. The time periods for Sublessor's and Prime Lessor's
               review and consent to any proposed assignment or subletting by
               Sublessee, as provided in Section 8.04 of the Prime Lease, shall
               run concurrently.

               (b)  If Sublessor shall consent to an assignment or subletting,
               without implying or requiring the granting of such consent, then
               one half (1/2) of any excess consideration payable to Sublessee
               as a result of such assignment or sublease (less brokerage
               commissions, attorneys' fees and other disbursements reasonably
               incurred by Sublessee for such assignment and subletting) shall
               be payable to Sublessor.

               (c)  If an default occurs after an assignment or subletting, in
               addition to any other remedies available to Sublessor hereunder
               or provided by law, Sublessor may at its option collect directly
               from the assignee or sublessee all rents payable to Sublessee
               under the assignment or sublease and apply the sums so received
               against any sums due to Sublessor under this Sublease. Sublessee
               authorizes and directs any assignee or sublessee to make such
               payments directly to Sublessor upon receipt of notice from
               Sublessor.  No direct collection of rent by Sublessor shall be a
               novation or a release of Sublessee from the performance of its
               obligations under this Sublease. Receipt by Sublessor of rent
               from any assignee or sublessee is not a waiver of the covenant
               against assignment and subletting or a release of Sublessee.

Prime Lessor's
Responsi-
bilities       12.  Sublessee recognizes that Sublessor is not in a position to
               furnish the services set forth in the Prime Lease, obtain an
               agreement of nondisturbance, or perform certain other obligations
               which are not within

                                     Page 7
<PAGE>

               the control of Sublessor, such as, without limitation,
               maintenance, repairs and replacements to the Building and
               Sublease Premises, compliance with laws, and restoration of the
               Premises and Building after casualty or condemnation. Therefore,
               notwithstanding anything to the contrary contained in this
               Sublease, Sublessee agrees that Sublessee shall look solely to
               Prime Lessor to furnish all services and to perform all
               obligations agreed upon by Prime Lessor under the Prime Lease to
               furnish and perform. Sublessor shall not be liable to Sublessee
               or be deemed in default hereunder for failure of Prime Lessor to
               furnish or perform the same. However, whenever under the terms of
               the Prime Lease, Prime Lessor shall fail to perform any of its
               Prime Lease obligations pertaining to the Sublease Premises,
               Sublessee may, at its option, enforce performance thereof if and
               to the extent authorized by the terms of the Prime Lease, and
               Sublessor shall cooperate with Sublessee in such enforcement.

Casualty and
Condem-
nation         13.  Article 15, entitled "Damage by Fire or Other Cause," and
               Article 16, entitled "Condemnation" of the Prime Lease are
               modified to provide that if by operation of either of these two
               provisions the Prime Lease is not terminated and continues in
               full force and effect, this Sublease shall not be terminated but
               shall also continue in full force and effect, except that until
               the Sublease Premises are restored in accordance with these two
               provisions there shall be a proportionate abatement of annual
               rent and additional rent payable hereunder to the extent of
               damage to the Sublease Premises as determined by Prime Lessor,
               Sublessor and Sublessee; provided, however, that such abatement
               shall in no event exceed the abatement granted to Sublessor under
               the Prime Lease  as to the Sublease Premises only, and, provided
               further, that no compensation or claim or reduction will be
               allowed or paid by Sublessor by reason of inconvenience,
               annoyance or injury to Sublessee's business arising from the
               necessity of effecting repairs to the Premises or any portion of
               the Building, whether such repairs are required by operation of
               these two provisions or any other provision of the Prime Lease.

Parking        14.  Sublessor hereby assigns Sublessor's parking spaces and
               parking rights under the Prime Lease to Sublessee, to use and
               enjoy during the term hereof.

Binding and

                                     Page 8
<PAGE>

Entire
Agreement      15.  This Sublease shall be binding on Sublessor and Sublessee
               and their respective successors and assigns. This Sublease
               contains the entire agreement of the parties with respect to the
               subject matter herein and may not be modified except by
               instrument in writing which is signed by both parties.

Consent of
Prime Lessor   16.  This Sublease is conditioned upon Prime Lessor's execution
               of a consent to this Sublease (the "Consent to Sublease") as
               required by the Prime Lease, in form reasonably satisfactory to
               Sublessor and Sublessee.

Broker
Agreement      17.  The parties warrant and represent to each other that no
               Person has negotiated or brought about this transaction other
               than The Pinnacle Group (who will be compensated according to a
               separate agreement with Sublessor) and Colliers Oxford
               Commercial, Inc. (who will be compensated according to a separate
               agreement with Sublessor) ("Broker"). No other or additional
               commissions shall be payable. Sublessee and Sublessor shall
               defend, indemnify, and save harmless one another from and against
               any claim which may be asserted against one of them by any Person
               other than the Person named above if the claim is made in
               connection with this transaction claiming under such party. This
               section shall survive the expiration or earlier termination of
               this Sublease.

Attorney's
Fees           18.  If it becomes necessary for either party to secure the
               services of an attorney in order to collect any amount due
               hereunder or to enforce any of the provisions hereof, the
               defaulting party agrees and binds itself to pay to the other
               reasonable attorney's fees and court costs.

General        19.  (a)  this Sublease Shall Be Construed and Interpreted under
               the Laws of the State of Texas and All Obligations of the Parties
               Hereunder Are Performable in Travis County, Texas.

               (b)  Time is of the essence in the performance of all obligations
               of the parties.

               (c)  Any number of counterparts of this Sublease may be executed
               by the parties hereto, and each executed counterpart shall be
               deemed an original agreement.

                                     Page 9
<PAGE>

     IN WITNESS WHEREOF, duly authorized representatives of the parties hereto
have executed this Sublease as of the day and year first above written.

[The remainder of this page is intentionally left blank.  Signatures appear on
the following signature pages, attached hereto and made a part hereof.]

                                    Page 10
<PAGE>

                                SIGNATURE PAGE
                                      TO
                              SUBLEASE AGREEMENT
                                    BETWEEN
                       APTIS, INC., d/b/a APTIS SOFTWARE
                                      AND
                             VIGNETTE CORPORATION


                                     SUBLESSOR:

                                     APTIS, INC., d/b/a APTIS SOFTWARE
WITNESS:

----------------------
                                      By:   /s/ David Tusa
----------------------                   --------------------------------------
                                      Name:     David Tusa
----------------------                    -------------------------------------
                                      Title:    Senior Vice President
                                                and Chief Financial Officer
                                           ------------------------------------



                                    Page 11
<PAGE>

                                SIGNATURE PAGE
                                      TO
                              SUBLEASE AGREEMENT
                                    BETWEEN
                       APTIS, INC., d/b/a APTIS SOFTWARE
                                      AND
                             VIGNETTE CORPORATION


                                          SUBLESSEE:

                                          VIGNETTE CORPORATION


WITNESS:

----------------------
                                      By:   /s/ Joel G. Katz
----------------------                   --------------------------------------
                                      Name:     Joel G. Katz
----------------------                    -------------------------------------
                                      Title:    Chief Financial Officer
                                           ------------------------------------


WITNESS:


                                    Page 12
<PAGE>

                                SIGNATURE PAGE
                                      TO
                              SUBLEASE AGREEMENT
                                    BETWEEN
                       APTIS, INC., d/b/a APTIS SOFTWARE
                                      AND
                             VIGNETTE CORPORATION


                                        GUARANTOR:

WITNESS:                                BILLING CONCEPTS CORP.

-------------------                     By:  /s/ David Tusa
                                           ---------------------------------
-------------------                     Name:    David Tusa
                                        Title:   Senior Vice Prsident
-------------------                              and Chief Financial Officer

                                    Page 13