You can only add 100 EVs to each stat with vitamins, though.
In the 1st and 2nd Gen, you can put 255 EVs into every stat.
So if you use ten of each vitamin and Rare Candy it to Level 100, it won't be as powerful as a a Pokemon who was trained the "Normal" Way.
So don't max out your Level with Daycare or Rare Candies after using your vitamins. With fighting Wild Pokemon and Other trainers, you can max out your Pokemon's stats all the way.
Actually, the first and second gen use Stat experience instead of EVs. They are similar in concept, but there is no cap for stat experience. As such, Vitamins can be used at any point in the pokemon's growth, and there wouldn't be any severe negative repurcussions. I believe there is a point that vitamins can't be taken, but I can't recall what it is off the top of my head. >_>