My dog needs a vitamin supplement on top of his commercial dog food.

As long as your pet is consuming a commercial food appropriate for its life stage which carries a Nutritional Complete and Balanced Adequacy Statement provided by the Association of American Feed Control Officials (AAFCO), there is no need to give an additional vitamin or mineral supplement. Vitamin and mineral supplements marketed toward pets often assume you are also feeding a complete and balanced pet food, therefore, contain very low levels of vitamins and minerals in order to avoid potential toxicities.  Save your money for more important pet related expenses.

If you have more specific questions - Ask the Nutritionist.