Hre is another site, and part of it says:
Advantages of Preparing Your Own Dog Food at Home
There are many benefits of homemade dog food. Firstly, it is free of the additives, flavorings and contaminants often found in industrial pet food. Furthermore, it allows you to know exactly what goes into your dog’s food and what doesn’t – which is important, especially when considering what's in commercial pet food and how it is made. If its diet is well balanced, most dogs can thrive on a purely vegetarian diet, too. However, you should consult your veterinarian before switching your dog to an exclusively vegetarian diet.
Vegetarian Dog Food Recipes: Homemade Meatless Meals for your Pet | Suite101.com
There are many, many sites covering vegetarian diets for dogs.