How big of a forum? Is it maybe 15 users? Or more along the lines of 15000 users. Or is it a brand new forum that has no users at all yet, and what is it about? We need to know to make estimates of how much space and bandwidth are required.
Also, VBulletin is paid license software. If you can afford a key for it, you probably should just get paid hosting. If not, it possible that somebody will sponsor a key for you- however don't count on it.
Any PHP/MySQL server combo that is set up in a sane manner can support VBulletin, the issue is the licensing for it.