Not only a waste of money, but some vitamins, the fat soluble type, can be over dosed on and damaging. Too much vitamin A can destroy the liver and can kill. Also things like selenium.....I forget what else can also be harmful if over dosed on. So, when they put these suppliments, as they do in the high quality kibbles, I'd want to know exactly how much the dog is getting from the kibble before adding vitamin/mineral suppliments. Added together, if the amount is appropriate, then it would be OK. But to arbitrarily give addiditional suppliments could be dangerous.
I know....these vets. Any vet I've ever mentioned any of several high, premium foods, look absolutely blank. They only know about Science Diet. LOL.