PETA (People for the Ethical Treatment of Animals) and VIVA should be able to advise you on the best combinations and amounts to keep a veggie dog healthy throughout the different stages of his/her life.
As I understand it, many vegetarian dogs can thrive. The longest living dog from the Guiness Book of Records was a vegetarian dog. But as an aside, nobody should try and force vegetarianism on a cat. They definitely require meat in their diet - trust them to be awkward huh?