You can't. Once he has enough stat experience in defense, you can't feed him anymore iron. You'll have to defeat pokemon to increase the stat experience to the maximum now. --- http://xlittlewingx.deviantart.com
However, any Pokemon that's been in battles, will have Stat Experience, which reduces the amount of vitamins it can use. This isn't a penalty - It just means that the Pokemon's gained Stat experience equal to the amount of Vitamins it's missed out on prior to "feeding".
Regardless of when your Pokemon stops taking Vitamins, you'll still have the same amount of battles to go before you fully max that Pokemon's Stat experience.
Vitamins should only be classed as a shortcut for training your Pokemon, not a necessity. They are expensive, and are best used on an untrained Pokemon (if you wish to use the full 10 for the respective stat, that is).
The Pokemon will still need about 20 runs, solo against the Elite 4 (IIRC) to max out Stat Experience fully when it's unable to take any more Vitamins. --- Where logic won't work - I will PAL Gamers unite - http://www.gamefaqs.com/boards/277-pal-gaming