There are 6 Stats since GSC. In the 3rd Gen Games, ALL Stats have their own modifier. Special Attack and Special Defense don't share the same Special modifier anymore (A meassure made in GSC to allow trading with RBY...).
In short, 5 of the Stats can get 10 Vitamins, while the remaining Stat can only get one. Say you give 10 Vitamins to all Stats except HP. If you do that, you can only feed 1 HP Up to your Pokémon... it's simple...