Mix together the beef, bread crumbs, onion, egg, salt, pepper, and 1/2 cup of the tomato sauce. Form this mixture into a loaf and place it in a shallow pan. Stir together the remaining tomato sauce, vinegar, sugar, mustard, Worcestershire and the water (if too thick). Pour this sauce over the meatloaf. Bake for 1 hour, basting every 15 minutes with the pan juices.