I wouldn't do vitamins on empty. Plus vitamins are designed to work with your food to be more effective. Sometimes getting your appetite back just means eating when you don't really want to. Your body still requires energy. If I've learned anything, just because you don't "feel" like it, doesn't mean you dont' need food....and a consistent flow of it. even if you just grab an apple or yogurt those times you don't feel like eating.
I know the feeling and what you're going through. Happens to me too. Try to make an effort just to eat regardless SOMEthing every 3ish hours