Here is a good article on vitamin toxicity in dogs:
Fat Soluble Vitamins: Vitamin A, D, E, & K in Dogs
Purina Pro-Plan meets nutritional requirements for dogs, so as long as your dog is eating a healthy amount of food, you shouldn't need a vitamin supplement. I wasn't able to find an ingredient list for the Natures Farmacy's "Dogzymes Complete" linked to above, but it's doubtful that it would poison your dog if you give the recommended amount of the supplement; plus some of the other ingredients like probiotics, glucosamine, and chondroitin might be beneficial. It's really a question of whether you want to pay for something that might not be necessary.