I'm not going to vote, I cant bring myself to vote on these things...I've used both of them, and have used them fairly extensively...IPB seems to have a better admin side, whereas vB seems to have a better user-side...It all just comes down to personal preference in my book. Other two things you might want to consider...vB has a larger mod/support community, but IPB has a slightly better pricing scheme
That said, I use vB