I was under the impression that excess vitamin c was just excreted and therefore not toxic - I found this article which confirms that
Vitamin C (Ascorbic Acid)
Effects: Vitamin C is synthesized in the liver by dogs. It is important in collagen synthesis, and in many other metabolic reactions, including proper functioning of the immune system. Since the dog's body produces Vitamin C, many pet food manufacturers and veterinarians state that it is not required in the food and that oversupplementation may be harmful. This is not true, and supplementation in an appropriate form (for example Ester-C, calcium ascorbate) can have beneficial effects on dogs suffering from chronic joint and musculoskeletal disorders. In puppies it helps to prevent the development of such disorders.
Deficiency: Signs of deficiency include impaired wound healing, greater susceptibility to infection (decreased immune function), muscle and joint pain.
Toxicity: None, vitamin C is water soluble and excess is regularly excreted by the body, but overconsumption may cause diarrhea since vitamin C is a natural laxative. Recent studies have shown that excessively high doses of vitamin C may contribute to the formation of kidney stones, and in very rare cases anemia caused by interference with vitamin B12 absorption.
Stability: Partially or completely destroyed by overcooking or long periods of storage as it is sensitive to heat, light and oxygen.
Sources: Citrus fruits, berries, tomatoes, cauliflower, potatoes, green leafy vegetables, green peppers
