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):

vitiated
    adj 1: impaired by diminution [syn: diminished, lessened,
           vitiated, weakened]
    2: ruined in character or quality [syn: corrupted, debased,
       vitiated]