Why you want to pull the nature of this wonderful species?
Dog's ancestors are wolves and they are pure carnivorous animals. Evolution made a lot of variety of breeds but changing diet may effect its health and living time span.
Wild dogs are pure carnivorous also that meant the nature of dogs primarily ate meat products.
I strictly recommend you to not to make your dog pure veg and change his nature of diet. If you are veg then atleast feed your dog non-veg but don't ruin the nature of diet.