I have a dog that has been vegan (not just vegetarian) for almost three years now. His diet is however not due to any of my convictions, but due to an allergy he has against animal protein!
I have to say that he has never been healthier and fitter, he is doing very well on his diet (Benevo, which is a dry food, with Missing Link Vegetarian supplement and cold pressed nut oil for omega 3 and 6). He doesn't have to lose out on any treats as there are lots of vegan treats available these days. For training, I get vegan sausages, vegan chicken style treats, vegan cheese etc. which he all loves!
So although I wouldn't chose to feed a dog on a vegetarian/vegan diet just for the sake of it, I can honestly say that if done correctly, they can be just as healthy and fit!
Here's our vegan dog Flash:
With his TTouch practitioner Jacqui to show you how shiny his fur is
On holiday in action