Vegito is canon because he exists in the Manga (main canon of Dragon Ball). Gogeta doesn't exist in the Manga and only exists in the Anime and Movies which are considered alternate versions of Dragon Ball from the Manga therefore not canon.
Gogeta may not be canon but he can technically. The fusion dance is canon since goten and trunks do it in the majin buu saga and even in dragon ball super. That dance (obviously) fused goten and trunks into gotenks