It's more like they evolve similar to a Pokemon the more they eat as vegan the higher their level gets the record highest level ever recorded is 9 where they can't eat anything that has a definable shape but some have claimed to have known a level 10. Their is also the rare exceptions when people have allergies they require a leaf stone to evolve.
i'm a level 101 vegan, I refuse to eat anything that exists on this plane of reality