The Collaborative International Dictionary of English v.0.48:

Vitiate \Vi"ti*ate\, v. t. [imp. & p. p. Vitiated; p. pr. &
   vb. n. Vitiating.] [L. vitiatus, p. p. vitiare to vitiate,
   fr. vitium a fault, vice. See Vice a fault.] [Written also
   viciate.]
   1. To make vicious, faulty, or imperfect; to render
      defective; to injure the substance or qualities of; to
      impair; to contaminate; to spoil; as, exaggeration
      vitiates a style of writing; sewer gas vitiates the air.
      [1913 Webster]

            A will vitiated and growth out of love with the
            truth disposes the understanding to error and
            delusion.                             --South.
      [1913 Webster]

            Without care it may be used to vitiate our minds.
                                                  --Burke.
      [1913 Webster]

            This undistinguishing complaisance will vitiate the
            taste of readers.                     --Garth.
      [1913 Webster]

   2. To cause to fail of effect, either wholly or in part; to
      make void; to destroy, as the validity or binding force of
      an instrument or transaction; to annul; as, any undue
      influence exerted on a jury vitiates their verdict; fraud
      vitiates a contract.
      [1913 Webster]

WordNet (r) 3.0 (2006):

vitiate
    v 1: corrupt morally or by intemperance or sensuality; "debauch
         the young people with wine and women"; "Socrates was
         accused of corrupting young men"; "Do school counselors
         subvert young children?"; "corrupt the morals" [syn:
         corrupt, pervert, subvert, demoralize,
         demoralise, debauch, debase, profane, vitiate,
         deprave, misdirect]
    2: make imperfect; "nothing marred her beauty" [syn: mar,
       impair, spoil, deflower, vitiate]
    3: take away the legal force of or render ineffective;
       "invalidate a contract" [syn: invalidate, void,
       vitiate] [ant: validate]

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "vitiate":
   abate, abolish, abrogate, adulterate, alloy, annihilate, annul,
   bastardize, blemish, bring to nothing, brutalize, buffer, cancel,
   cancel out, canker, cheapen, coarsen, come to nothing, confound,
   contaminate, corrupt, corrupted, counterbalance, damage, debase,
   debauch, debauched, defile, deflower, degenerate, degrade, delete,
   demoralize, denature, deprave, depraved, depreciate, depress,
   desecrate, despoil, destroy, devalue, diminish, distort, downgrade,
   frustrate, harm, hurt, impair, infect, invalidate, lower, mar,
   misuse, negate, negativate, negative, neutralize, nullify, offset,
   pervert, perverted, poison, pollute, prejudice, prostitute, quash,
   ravage, ravish, reduce, revoke, ruin, soil, spoil, stultify, sully,
   suppress, taint, tarnish, thwart, twist, ulcerate, undermine, undo,
   violate, vitiated, void, vulgarize, warp, weaken, withdraw