To reduce the value or quality of; impair or spoil:"His famous compilation of norms was vitiated by a major sampling error"(Frederick Crews).
To corrupt morally; debase:"My anxieties .. still are great lest the numerous .. snares of vice should vitiate your early habits of virtue"(Abigail Adams). See Synonyms at corrupt.
To make ineffective (a contract or legal stipulation, for example); invalidate.