I deeply disapprove of vegetarian, much less vegan, diets for dogs and cats. Imposing such an utterly unnatural diet on one's helpless beasts strikes me as the height of arrogance. And I wouldn't be surprised if it hurts the animals as well. Cats, in particular, but even dogs. In fact, many vegans/vegetarians feed their beasts raw, setting aside their own notions about the morality of consuming meat and other animal products when it comes to their beasts on the reasoning that it is best to feed them what they would be consuming in nature. No cat or dog would ever be a vegetarian/vegan by choice.