Oh. Gen 4 is different. Lvl 100 Pokemon don't gain EVs in those games, at least not from battling. Vitamins should work just fine though, but only to 100 EVs in any given field.
In Gen 5, EVs are calulated after every battle, rather than just when you level up, and finishing a battle is the only requirement to gain EVs, allowing Lvl 100 Pokemon to be EV Trained.