That's becasue your Electivire already has too many EV's. Vitamins only work when the stat's EV's are less than 100. Since you used him in game he has picked up random EV's all over the place. The only way you can properly train him is to feed him the "happy" berries until you get a message saying the stat won't go any lower. These are Pomeg, Kelpsy, Grepa, Hondew, Tamato and some other berry, which lower the EV's of a certain stat by 10. If you feed him each of these constantly until each one gives you the above message, then his EV's will drop to 0 and you can train him from scratch.