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]