To vitiate something is to ruin it or render it ineffective.

// A single inaccuracy in the spreadsheet that supported the data vitiated the entire proposal.

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"… Lily Gladstone does more with thought, in 'Killers of the Flower Moon,' than most actors ever achieve with flagrant and spectacular action; her presence fills the screen with what used to be understood as star power (before the mainly technical prowess of conservatory-trained actors became a mark of Hollywood dignity). The role she plays is one of tragic complexity; blatant theatrics would have vitiated its grandeur." — Richard Brody, The New Yorker, 23 Jan. 2024

If you’re itching to find fault with the word vitiate, you don’t have to look far—the word comes ultimately from the Latin noun vitium, which refers to a fault, vice, shortcoming, or impediment to success or perfection. Accordingly, vitiate—like its fellow vitium-descended v-words vituperate ("to use harsh, condemning language"), vicious, and vice—has a negative bent. To vitiate something is, essentially, to mar or damage it in some way, whether by ruining or spoiling it ("a joke vitiated by poor timing"), corrupting it morally ("a mind vitiated by prejudice"), or rendering it null or ineffective ("fraud that vitiates a contract"). Despite its versatility, vitiate is most effective when used in formal speech and writing; that is to say, those who drop it into a construction like "a sandwich vitiated by too much peanut butter" may find themselves subject to some mild vituperation.