Medicine

Protein raises the Pokemon that you used the vitamin on's base attack stat. It only raises the stat a little bit, so unless the Pokemon that you want to train is on a high level, then you should still be able to Ev train him in whatever way. The vitamins that you can use on a Pokemon don't do anything other than raise the stat that they have been designed to. Try again, but with a lower level Pokemon. Or, you could just not give him as many proteins this time...

Hope this helps.

Protein and other stat-uping items give a basic one point boost to the main stat that they effect. You can use up to ten of each. That leaves twenty one more slots for EV points to go in and boost main stats. There should be an EV guide somewhere in the Diamond Game Index.... Sorry, but I can't remember the names of the Pokemon and what EVs they give.

You can EV train till around 40-45, because after that it starts to get hard to level up. See, when battling a certain Pokemon, (again, sorry I forgot) upon victory, if you level up, the Pokemon may give EV points. The only examples I can think of right now are Ghastly and Geodude... But the table should have it.

Here is the link to the game index: http://www.supercheats.com/nintendods/pokemondiamond.htm

You see, each of the medicine like Protein, Iron, etc. Each up a certain stat. On your Pokemon, you can use about up to 10 of each stat inhancer.