I am a vegetarian, and I have thought about this issue and have sadly not come to a conclusion yet. My cats are on Wysong, and I know they do make a canine/feline diet that is vegan... but I'm not sure if I want them on that. Not only may they not eat it, but cats were not meant to eat vegetables.
I guess, for me, having them on one of the higher quality brands of pet foods makes a difference because maybe that brand gets their meat from a 'better' source. That make sense?