Here are some guidelines for feeding vegetarian dogs.
Vegetables are highly digestible by dogs after cooking. Baking is best for nutrient retention; but boiling is fine, if you also use the water in the food. Dogs love most cooked vegetables, but may prefer them cut into small pieces or pureed. Whole cooked potatoes make an excellent snack. Just fill a pan with potatoes and place in the oven for about an hour at 350 degrees. After cooling, store in the refrigerator and give as snacks. As much as 1/3 of the basic recipe can consist of "extra" vegetables without greatly altering the nutrient balance.
Millet, beans, lentils, and other grains/legumes.
Add up to one cooked cup per basic recipe. Beans, lentils and other legumes are especially high in minerals and protein. However, as mentioned earlier, you should check the dog(s) stool to see if they are digesting such foods. If you see whole beans, for example, cook them longer and/or put them in a blender.
Add up to one avocado per basic recipe. This is an excellent source of essential fatty acids, among other important nutrients. The addition of one avocado will lower the protein requirement slightly to 15% (and increase the fat content). However, this amount is still greater than that suggested by one veterinarian.4
Add up to 1 cup dry or cooked pasta to the food. Any size shape or variety will do-- it should be whole wheat or enriched, however.
Whole wheat bread
Dogs love bread. Fortunately, bread is a healthy snack. You can add it to the meal or use for treats.