I don't have a recommendation for vitamins but recently my son went through a period where all he would eat was spaghetti. I added finely chopped veggies (green pepper, celery, onion) to the sauce. I also added baby food (carrots, beans, peas, sweet potatoes) to the sauce - one kind at a time. He never noticed that I added anything.
I also got him to "drink" Pediapals. I had to give it to him through a medicine syringe because he wouldn't drink it outright. I also told him it was medicine that the doctor said would make him better (he is on a long term medication regimen so he didn't think much of it). I would be able to get an entire bottle in him by the end of the day.
A friend had recommended making "smoothies" out of ice cream or yogurt and adding pediapals or breakfast drink and also pureed fruit.
Good luck!