There’s no such thing, strictly speaking, as correctness in the use of English. There is only received form. Nevertheless, it troubles me slightly when somebody writes ‘His friend was the only person he could talk to.’ Being a purist who likes to have prepositions in their proper place, I would prefer ‘His friend was the only person to whom he could talk.’ But there’s no such thing as purity either. When Sherlock Holmes reads a letter written in English by an anonymous writer whom Holmes has calculated to be the King of Bohemia, he remarks: ‘The writer must be a German speaker. Only a German would be so disrespectful to his verbs.’ Quite so, but it hardly matters.
A person might write ‘incorrect’ English out of ignorance or choice. The former betrays a lack of knowledge, the latter proclaims a selection of style. Sometimes we change our style to suit the sense of the moment. I do, and I’m sure Vonnegut knew what he was doing just as much as Nabokov did. In Lolita, Nabokov’s style is that of a fine vintage brandy; in Slaughterhouse 5, Vonnegut’s is redolent of a strawberry milkshake. Mostly I prefer the brandy, but they both taste good.