First off, let me say that this comment made me google whether or not a dog can sustainably be vegan, and it is possible. My initial thoughts were incorrect. However, that is not to say it's what is optimal for the animal and if we care about our animals, we shouldnt add dietary stress to their lives. Per PetWebMD: "For dogs, certainly vegetarian and vegan diets can be done, but they need to be done very, very carefully. There is a lot of room for error, and these diets probably are not as appropriate as diets that contain at least some animal protein,"

Also you conveniently chose not to discuss the other predominant domesticated animal, Cats, which a vegan diet would be completely unhealthy diet. “For cats, it’s really inappropriate. It goes against their physiology and isn’t something I would recommend at all," says Cailin Heinze, VMD, a board-certified veterinary nutritionist and assistant professor of nutrition at Tufts Cummings School of Veterinary Medicine.

Source: http://pets.webmd.com/features/vegetarian-diet-dogs-cats#1