I have to say, I have seen first hand some of the vitiation you say doesn't happen.
From the economist.com
Actually, vitiation is probably too strong a word for it.
From the economist.com
One senses vitiation and fatigue, an artist responding to the surface of a changing world, rather than leading the conversation.
From the washingtonpost.com
If the theatre-going public accepts this vitiation of its entertainment program a deplorable decline in the Art of Music is inevitable.
From the en.wikipedia.org
Just by observing the Laus'behaviour, and consideration of the situation before signing, there was no coercion amounting to a vitiation of consent.
From the en.wikipedia.org
Calatrava's appealing construct only emphasized the emptiness of the ballet it supported, the vitiation of the language of classical ballet that Millepied struggled to reanimate.
From the washingtonpost.com
The Laus'considered the situation before signing, and did not behave like someone under duress, so there was no coercion amounting to a vitiation of consent.
From the en.wikipedia.org
More examples
Nullification by the destruction of the legal force; rendering null; "the vitiation of the contract"
(vitiate) corrupt: 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"
(vitiate) mar: make imperfect; "nothing marred her beauty"
(vitiate) invalidate: take away the legal force of or render ineffective; "invalidate a contract"
(vitiated) diminished: impaired by diminution
(vitiated) corrupted: ruined in character or quality
A reduction in the value, or an impairment in the quality of something; moral corruption; an abolition or abrogation
(vitiate) to spoil, make faulty; to reduce the value, quality, or effectiveness of something; to debase or morally corrupt; to violate, to rape; to make something ineffective, to invalidate
(Vitiate) To impair or make void; to destroy or annul, either completely or partially, the force and effect of an act or instrument.