A vanilla game is one where you have nothing to do with cuckoldry or any other obscure and niche fetishes (scat, bestiality, etc.)
A vanilla path should be able to stand by itself and have enough scenes whose purpose isn't to emasculate the MC or prove him worthy of being cucked. Playing the vanilla path shouldn't be about preventing the ntr, nor should it just be herding the girl away from everyone since she cannot be trusted.