Document:

<PAGE>   1
                                                                   EXHIBIT 4.8.1

                  DECLARATION OF TRUST, dated as of March 22, 2000 between
Valero Energy Corporation, a Delaware corporation, as Sponsor (the "Sponsor"),
and The Bank of New York, as trustee (the "Property Trustee"), The Bank of New
York (Delaware), as trustee (the "Delaware Trustee"), and Jay D. Browning, as
trustee (the "Regular Trustee") (the Property Trustee, the Delaware Trustee and
the Regular Trustee, collectively the "Trustees"). The Sponsor and the Trustees
hereby agree as follows:

                  1. The Delaware business trust created hereby shall be known
as "VEC Trust I" (the "Trust"), in which name the Trustees, or the Sponsor to
the extent provided herein, may conduct the business of the Trust, make and
execute contracts, and sue and be sued.

                  2. The Sponsor hereby assigns, transfers, conveys and sets
over to the Trust the sum of $10. The Trust hereby acknowledges receipt of such
amount from the Sponsor, which amount shall constitute the initial trust estate.
It is the intention of the parties hereto that the Trust created hereby
constitute a business trust under Chapter 38 of Title 12 of the Delaware Code,
12 Del. Code Section 3801 et seq. (the "Business Trust Act"), and that this
document constitutes the governing instrument of the Trust. The Trustees are
hereby authorized and directed to execute and file a certificate of trust with
the Secretary of State of the State of Delaware in the form attached hereto in
accordance with the provisions of the Business Trust Act. The Trust is hereby
established by the Sponsor and the Trustees for the purposes of (i) issuing
preferred securities representing undivided beneficial interests in the assets
of the Trust ("Preferred Securities") in exchange for cash and investing the
proceeds thereof in debt securities of the Sponsor, (ii) issuing and selling
common securities representing an undivided beneficial interest in the assets of
the Trust ("Common Securities") to the Sponsor in exchange for cash and
investing the proceeds thereof in additional debt securities of the Sponsor and
(iii) engaging in such other activities as are necessary or incidental thereto.

                  3. The Sponsor and the Trustees will enter into an Amended and
Restated Declaration of Trust, satisfactory to each such party and substantially
in the form included as an exhibit to the Securities Act Registration Statement
referred to below, to provide for the contemplated operation of the Trust
created hereby and the issuance of the Preferred Securities and Common
Securities referred to therein. Prior to the execution and delivery of such
Amended and Restated Declaration of Trust, the Trustees shall not have any duty
or obligation hereunder or with respect to the trust estate, except as otherwise
required by applicable law or as may be necessary to obtain prior to such
execution and delivery any licenses, consents or approvals required by
applicable law or otherwise.

                  4. The Sponsor, as the sponsor of the Trust, is hereby
authorized (i) to prepare and file with the Securities and Exchange Commission
(the "Commission") and execute, in each case on behalf of the Trust, (a) a
Registration Statement on Form S-3 or on such other form or forms as may be
appropriate, including without limitation any registration statement of the type
contemplated by Rule 462(b) of the Securities Act of 1933, as amended (the
"Securities Act") (any such registration statement, whether on Form S-3, another
form or under Rule 462(b) being referred to herein as the "Securities Act
Registration Statement"), including any pre-effective or post-effective
amendments to such Registration Statement, relating to the registration under
the Securities Act of the Preferred Securities and certain other securities of
the Sponsor and (b) if the Sponsor shall deem it desirable, a Registration
Statement on Form 8-A (the

<PAGE>   2

"Exchange Act Registration Statement"), including all pre-effective and
post-effective amendments thereto, relating to the registration of the Preferred
Securities under Section 12 of the Securities Exchange Act of 1934, as amended;
(ii) if the Sponsor shall deem it desirable, to prepare and file with New York
Stock Exchange, Inc. or any other automated quotation system, exchange or
over-the-counter market (collectively, the "Exchanges") and execute on behalf of
the Trust a listing application or applications and all other applications,
statements, certificates, agreements and other instruments as shall be necessary
or desirable to cause the Preferred Securities to be listed on any Exchange;
(iii) to prepare and file and execute on behalf of the Trust such applications,
reports, surety bonds, irrevocable consents, appointments of attorney for
service of process and other papers and documents as the Sponsor, on behalf of
the Trust, may deem necessary or desirable to register the Preferred Securities
under the securities or "Blue Sky" laws of such jurisdictions as the Sponsor, on
behalf of the Trust, may deem necessary or desirable; (iv) to negotiate the
terms of and execute on behalf of the Trust an underwriting or other purchase
agreement among the Trust, the Sponsor and any underwriter(s), dealer(s) or
agent(s) relating to the Preferred Securities, as the Sponsor, on behalf of the
Trust, may deem necessary or desirable; and (v) to execute and deliver on behalf
of the Trust letters or documents to, or instruments for filing with, a
depository relating to the Preferred Securities. In the event that any filing
referred to in clauses (i)-(iii) above is required by the rules and regulations
of the Commission, any Exchange, the National Association of Securities Dealers,
Inc. or state securities or blue sky laws, to be executed on behalf of the Trust
by a Trustee, any natural person appointed pursuant to Section 6 hereof, in his
or her capacity as trustee of the Trust, and the Sponsor are hereby authorized
to join in any such filing and to execute on behalf of the Trust any and all of
the foregoing.

                  5. This Declaration of Trust may be executed in one or more
counterparts.

                  6. The number of Trustees initially shall be three (3) and
thereafter the number of Trustees shall be such number as shall be fixed from
time to time by a written instrument signed by the Sponsor that may increase or
decrease the number of Trustees; provided, however, that the number of Trustees
shall in no event be less than three (3); and provided, further that to the
extent required by the Business Trust Act, one Trustee shall either be a natural
person who is a resident of the State of Delaware or, if not a natural person,
an entity that has its principal place of business in the State of Delaware and
meets other requirements imposed by applicable law. Subject to the foregoing,
the Sponsor is entitled to appoint or remove without cause any Trustee at any
time. Any Trustee may resign upon thirty days' prior notice to the Sponsor.

                  7. The Trust may be dissolved and terminated at the election
of the Sponsor.

                  8. This Declaration of Trust shall be governed by, and
construed in accordance with, the laws of the State of Delaware (without regard
to conflict of laws principles).

                                       2
<PAGE>   3

                  IN WITNESS WHEREOF, the parties hereto have caused this
Declaration of Trust to be duly executed as of the day and year first above
written.

                                   VALERO ENERGY CORPORATION,
                                   as Sponsor

                                   By: /s/ Jay D. Browning
                                       --------------------------------------
                                       Jay D. Browning, Secretary

                                   THE BANK OF NEW YORK,
                                   as Property Trustee

                                   By:  /s/ Remo J. Reale
                                        -------------------------------------
                                        Name:  Remo J. Reale
                                        Title:  Vice President

                                   THE BANK OF NEW YORK (DELAWARE),
                                   as Delaware Trustee

                                   By:  /s/ John Nichols
                                        -------------------------------------
                                        Name:  John Nichols
                                        Title:  President

                                   JAY D. BROWNING,
                                   as Regular Trustee

                                   /s/ Jay D. Browning
                                   ------------------------------------------<PAGE>   1
                                                                   EXHIBIT 4.8.2

                  DECLARATION OF TRUST, dated as of March 22, 2000 between
Valero Energy Corporation, a Delaware corporation, as Sponsor (the "Sponsor"),
and The Bank of New York, as trustee (the "Property Trustee"), The Bank of New
York (Delaware), as trustee (the "Delaware Trustee"), and Jay D. Browning, as
trustee (the "Regular Trustee") (the Property Trustee, the Delaware Trustee and
the Regular Trustee, collectively the "Trustees"). The Sponsor and the Trustees
hereby agree as follows:

                  1. The Delaware business trust created hereby shall be known
as "VEC Trust II" (the "Trust"), in which name the Trustees, or the Sponsor to
the extent provided herein, may conduct the business of the Trust, make and
execute contracts, and sue and be sued.

                  2. The Sponsor hereby assigns, transfers, conveys and sets
over to the Trust the sum of $10. The Trust hereby acknowledges receipt of such
amount from the Sponsor, which amount shall constitute the initial trust estate.
It is the intention of the parties hereto that the Trust created hereby
constitute a business trust under Chapter 38 of Title 12 of the Delaware Code,
12 Del. Code Section 3801 et seq. (the "Business Trust Act"), and that this
document constitutes the governing instrument of the Trust. The Trustees are
hereby authorized and directed to execute and file a certificate of trust with
the Secretary of State of the State of Delaware in the form attached hereto in
accordance with the provisions of the Business Trust Act. The Trust is hereby
established by the Sponsor and the Trustees for the purposes of (i) issuing
preferred securities representing undivided beneficial interests in the assets
of the Trust ("Preferred Securities") in exchange for cash and investing the
proceeds thereof in debt securities of the Sponsor, (ii) issuing and selling
common securities representing an undivided beneficial interest in the assets of
the Trust ("Common Securities") to the Sponsor in exchange for cash and
investing the proceeds thereof in additional debt securities of the Sponsor and
(iii) engaging in such other activities as are necessary or incidental thereto.

                  3. The Sponsor and the Trustees will enter into an Amended and
Restated Declaration of Trust, satisfactory to each such party and substantially
in the form included as an exhibit to the Securities Act Registration Statement
referred to below, to provide for the contemplated operation of the Trust
created hereby and the issuance of the Preferred Securities and Common
Securities referred to therein. Prior to the execution and delivery of such
Amended and Restated Declaration of Trust, the Trustees shall not have any duty
or obligation hereunder or with respect to the trust estate, except as otherwise
required by applicable law or as may be necessary to obtain prior to such
execution and delivery any licenses, consents or approvals required by
applicable law or otherwise.

                  4. The Sponsor, as the sponsor of the Trust, is hereby
authorized (i) to prepare and file with the Securities and Exchange Commission
(the "Commission") and execute, in each case on behalf of the Trust, (a) a
Registration Statement on Form S-3 or on such other form or forms as may be
appropriate, including without limitation any registration statement of the type
contemplated by Rule 462(b) of the Securities Act of 1933, as amended (the
"Securities Act") (any such registration statement, whether on Form S-3, another
form or under Rule 462(b) being referred to herein as the "Securities Act
Registration Statement"), including any pre-effective or post-effective
amendments to such Registration Statement, relating to the registration under
the Securities Act of the Preferred Securities and certain other securities of
the Sponsor
<PAGE>   2
and (b) if the Sponsor shall deem it desirable, a Registration Statement on Form
8-A (the "Exchange Act Registration Statement"), including all pre-effective and
post-effective amendments thereto, relating to the registration of the Preferred
Securities under Section 12 of the Securities Exchange Act of 1934, as amended;
(ii) if the Sponsor shall deem it desirable, to prepare and file with New York
Stock Exchange, Inc. or any other automated quotation system, exchange or
over-the-counter market (collectively, the "Exchanges") and execute on behalf of
the Trust a listing application or applications and all other applications,
statements, certificates, agreements and other instruments as shall be necessary
or desirable to cause the Preferred Securities to be listed on any Exchange;
(iii) to prepare and file and execute on behalf of the Trust such applications,
reports, surety bonds, irrevocable consents, appointments of attorney for
service of process and other papers and documents as the Sponsor, on behalf of
the Trust, may deem necessary or desirable to register the Preferred Securities
under the securities or "Blue Sky" laws of such jurisdictions as the Sponsor, on
behalf of the Trust, may deem necessary or desirable; (iv) to negotiate the
terms of and execute on behalf of the Trust an underwriting or other purchase
agreement among the Trust, the Sponsor and any underwriter(s), dealer(s) or
agent(s) relating to the Preferred Securities, as the Sponsor, on behalf of the
Trust, may deem necessary or desirable; and (v) to execute and deliver on behalf
of the Trust letters or documents to, or instruments for filing with, a
depository relating to the Preferred Securities. In the event that any filing
referred to in clauses (i)-(iii) above is required by the rules and regulations
of the Commission, any Exchange, the National Association of Securities Dealers,
Inc. or state securities or blue sky laws, to be executed on behalf of the Trust
by a Trustee, any natural person appointed pursuant to Section 6 hereof, in his
or her capacity as trustee of the Trust, and the Sponsor are hereby authorized
to join in any such filing and to execute on behalf of the Trust any and all of
the foregoing.

                  5. This Declaration of Trust may be executed in one or more
counterparts.

                  6. The number of Trustees initially shall be three (3) and
thereafter the number of Trustees shall be such number as shall be fixed from
time to time by a written instrument signed by the Sponsor that may increase or
decrease the number of Trustees; provided, however, that the number of Trustees
shall in no event be less than three (3); and provided, further that to the
extent required by the Business Trust Act, one Trustee shall either be a natural
person who is a resident of the State of Delaware or, if not a natural person,
an entity that has its principal place of business in the State of Delaware and
meets other requirements imposed by applicable law. Subject to the foregoing,
the Sponsor is entitled to appoint or remove without cause any Trustee at any
time. Any Trustee may resign upon thirty days' prior notice to the Sponsor.

                  7. The Trust may be dissolved and terminated at the election
of the Sponsor.

                  8. This Declaration of Trust shall be governed by, and
construed in accordance with, the laws of the State of Delaware (without regard
to conflict of laws principles).

                                       2
<PAGE>   3

                  IN WITNESS WHEREOF, the parties hereto have caused this
Declaration of Trust to be duly executed as of the day and year first above
written.

                                   VALERO ENERGY CORPORATION,
                                   as Sponsor

                                   By: /s/ Jay D. Browning
                                       --------------------------------------
                                       Jay D. Browning, Secretary

                                   THE BANK OF NEW YORK,
                                   as Property Trustee

                                   By:  /s/ Remo J. Reale
                                        -------------------------------------
                                        Name:  Remo J. Reale
                                        Title:  Vice President

                                   THE BANK OF NEW YORK (DELAWARE),
                                   as Delaware Trustee

                                   By:  /s/ John Nichols
                                        -------------------------------------
                                        Name:  John Nichols
                                        Title:  President

                                   JAY D. BROWNING,
                                   as Regular Trustee

                                   /s/ Jay D. Browning
                                   ------------------------------------------

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