These are the basic interaction rules when figuring out when to take your vitamins:
Calcium and Vitamin D are Friends --- take them together, they help each other absorb better. Your body can only deal with 500mg at a time, so split up your doses into 3 or 4 doses per day to reach your 1500-2000mg daily goal. Calcium tends to absorb better when taken with a meal, so schedule it that way if you can.
Iron and Vitamin C are Friends --- Iron needs an acid environment to break down and Vitamin C does that job so make sure they are in your tummy at the same time. Iron does not like food, so take it on an empty stomach. However, if you get an upset tummy because of the iron, pick a non-dairy snack.
Iron and Calcium are Enemies --- iron and calcium fight for the same cell receptors in the body and calcium is bigger and badder and always wins. Which means the iron is simply excreted from the body and not used at all. Keep iron and calcium at least 2 hours apart from each other.
Vitamin B's are a Family --- they work together as a team and are best taken at the same time. Your Multi-Vitamin has many B's in it, so take it together with your biotin, B12 and B-complex if you're taking those too.