it depends on what is in the vitamins. the danger in too much vitamins lies mostly in the "fat soluable" vitamins (e, a, d). these are stored in your body rather than being flushed out by your kidneys. so vitamins with really high doses of a, d or e can become harmful to you. i don't believe cereals have those in them.
if you take too much water solouble vits (c, the b's to name 2) they are expelled through urine (which b vitamins turn a pretty color greeninsh yellow).
there are a lot of sites on the internet that can help you with this - just use you search engine as i don't think we're supposed to post sights here.