If you're feeding her the recommended levels of the feed you have now, then you don't need any extra vitamins. Just feed her free choice quality grass hay or mixed hay (or Timothy), provide a salt block, and plenty of turnout (preferably 24/7). Young horses need room to run and a friend or two, so their bones and tendons will strengthen and develop properly. Bring her in to eat once or twice a day, then turn her back out. Stalling youngsters is never a good idea.
Young horses go through growth spurts. It is completely normally for her to look "ribby" every once in a while when she hits a big one. Just increase her food a little or add some fat her diet and wait for her weight to catch up to her height. Be patient and don't over feed her. A fat baby can be detrimental to growing joints as well.