^ What Kansa said. Additionally, if your vet thinks that his puppy diet at the breeder caused some of the current issues, I would not likely listen to the breeder's advice on supplementing.
I personally don't "supplement" my dogs' food with vitamins. They only get things like fish oil, a joint supplement, and things like some hard boiled eggs, cottage cheese, sardines, etc. added to their food on a rotating basis. Their kibble is perfectly acceptable and they don't need any additional vitamins for it to be nutritionally complete.