vitiate \VISH-ee-ayt\ (transitive verb) - 1 : To make faulty or imperfect; to render defective; to impair; as, "exaggeration vitiates a style of writing." 2 : To corrupt morally; to debase. 3 : To render ineffective; as, "fraud vitiates a contract."
"It seems churlish to say of a book that is beautifully written, richly allusive, learned, elegant, Proustian in tone and mode, that precisely these qualities vitiate its ostensible purpose, distracting attention from the subject and focusing it upon the very gifted author." -- Gertrude Himmelfarb, 'A Man's Own Household His Enemies'
Vitiate comes from Latin vitiare , from vitium , fault. It is related to vice (a moral failing or fault ), which comes from vitium via French.