This is actually more complicated than it sounds. I have read several books on vitamins and did a couple experiments on uptake in college. My father takes B vitamins because they help with pain medication uptake.
I use to prescribe to the theory of taking what my body needed so I would place about 30 vitamin pills on the counter and sniff each bottle of pills. If they smelled sweet or like nothing at all I would take them, if it smelled bad it was a no go. Actually it did work in so far as my body telling me I did not need B-12 and if I took it I would be ill. But as it sounds this is a time consuming process and after a couple of months fell by the wayside.
When I got into pharmeceutical and we started exploring some of the nutraceutical end. I will warn you alot of vitamins have filler which you pee out or are made up of unusable forms by the body.
If you want to take them be fore warned also that exact recommendations for the heavy metals are not out there, or the findings are weak. The only for sure vitamin is C and you will pee out whatever your body can't use (also the body uses only the L-form and the vitamins are all mixtures, cost of extraction is too high). Too much vitamin A, Zinc, Chromium, and the like are not a good thing. Take your folic acid if you need it its another good one.
Like Kitty though I try to get most from my diet. BFL is very balanced so I feel good about that. If I do come down with a cold or something I will take up to 10 g a day of Vitamin C and sniff my B's for uptake. So there you go. Are you more confused?
If you want 1 answer taking a multi-vitamin gives the body things it doesn't need already forund in the diet. Also the body can come to expect it and not utilize your other natural intake as efficiently.