Depends on how they're applied. Lots of vitamins are water solluble, so unless you're using a food that absorbs liquids (dry pellets, etc.) most will just come off when you add the food to the tank. I'd personally try to inject shimp, silversides, earthworms, etc. with a syringe to make sure the fish are getting the vitamins.
But heck, I don't take vitamins myself, so I'm sure as heck not going to feed them to my fish. As long as they're fed variety they'll be fine.