Even granting that cats can be sustained on a diet of substances derived entirely by plants, cats can't actually be vegan. Veganism is a philosophy of what it means to be a person and how humans should live, which cats don't really have the brain structure to do. Even if you ignore that fact, cats are (I'm saying this as someone who loves cats) psychopathic little murderers; no matter how well-fed a cat is, it will still instinctively kill prey animals. Cats on the hunt will sometimes kill something to eat, see a new bit of prey, abandon their old kill, and hunt the new thing. Cats cannot be vegan.