We need to replace two elements: the person “Vincent” and the place “au carnaval.”
“Vincent” is a masculine proper noun, and we refer to him with the stressed pronoun “lui.”
“Au carnaval” is a location preceded by the preposition “à” — so we replace it with the adverbial pronoun “y.”
The verb “va” stays as is, since it refers to “tout le monde” (third person singular).
So the transformed sentence becomes: Selon lui, tout le monde y va.