You are doing just fine. Maybe try to stock up on those little containers of milk. Buy a few at a time and store them in your little fridge. Have you tried taking the vitamin with orange juice?
9 times out of 10 the baby takes what they need first, and you get what is left over. So mom's are the one's usually missing the vitamins :o)
If you eat a fairly balanced healthy diet, than you don't have too much to worry about.
If you have a chance (I know it's hard when you are a student to have a lot of time) go to your local health food store and try to find a multi-vitamin that is natural sourced. So the vitamins and minerals come from natural sources, then you will find it won't be so hard on your stomach or bowels.