This is a bad, bad no good idea.
First, vitamins are not made to be taken rectally. If they are, they are called suppositories. Vitamins are hard things. They need your stomach to break them down into little, absorbable chunks. But cramming a whole one up there, what is the point?
Look, it isn't like people who take vitamins are vitamin deficient and so have to lodge one up their ass to increase its absorbability. Just swallow the damn thing and leave room in your butt for other things.