I agree with Dean, most users don't care what software you use as long as it's easy to use and not full of bugs. If vB is slower that IPB then there is a problem with the server because I have found vB to be much better at handeling load than any of the free boards, especially since 3.0 has come out.
If there is a problem with the way the board is run you might want to visit
The Admin Zone for some help in getting it worked out. They have some really good information there.