This excerpt taken from the VLO 8-K filed Oct 26, 2005.
ARTICLE 8. AWARD PERIOD; VESTING
8.1 Award Period. Subject to the other provisions of this Plan, the Committee may, in its discretion, provide that an Incentive may not be exercised in whole or in part for any period or periods of time or beyond any date specified in the Award Agreement. Except as provided in the Award Agreement, an Incentive may be exercised in whole or in part at any time during its term.
No Incentive granted under the Plan may be exercised at any time after the end of its Award Period. The Plan limitations stated in Section 6.2(a)(vii) shall apply.
8.2 Vesting. The Committee, in its sole discretion, may determine that an Incentive will be immediately exercisable, in whole or in part, or that all or any portion may not be exercised until a date, or dates, subsequent to its Date of Grant, or until the occurrence of one or more specified events, subject in any case to the terms of the Plan. If the Committee imposes conditions upon exercise, then subsequent to the Date of Grant, the Committee may, in its sole discretion, accelerate the date on which all or any portion of the Incentive may be exercised.