You need to give us more info about your set. What version of vBulletin? Lots of mods? What webserver? What version of PHP? What are the server specs?
I'm going to take a wild guess and say that you're using Apache with PHP as a dso, but even with that I can't offer a lot of advice without knowing a bit more about your setup.
If the server is yours, you might check out
http://www.vbulletin.com/forum/forumdisplay.php?f=14 and ask for an optimization request.