"if we didn't have canine teeth and were able to get our necessary vitamin B12 from plants, his avowal for nothing but plant food would be more plausible."

B12 comes from bacteria, not animals. And about your canine teeth - nearly all mammals have canine teeth. That doesn't make them meat eaters.  Saber-toothed deer, gelada baboons, white-lipped peccaries, gorillas and hippopotamuses all have fearsome canines, and the largest in the world actually belong to hippos. Their canine teeth have been known to grow up to 18 inches, but play no role in how hippos gather food.  Camels - which are herbivores - have sharp little canine teeth. By contrast humans do not. It's a stretch to even call them canines. 

There are a whole bunch of reasons why humans are biologically more fit for a vegan diet than a meat-based diet.