There's a mod that puts how much money you need and how close you've gotten to your goal on your forum homepage (I forget what it is called, but I can get the link if you want it). Maybe that'll encourage your members to help out.
There are a couple, I think: AWCoding- Donation System - vBulletin.org Forum Donation Progress Bar-Forumhome - vBulletin.org Forum Cyb - PayPal Donate - vBulletin.org Forum
Rizzo, there's nothing wrong with straight-up asking your members for donations. It's the approach of doing so that is the key element. Saying "I need money for the site, pay now" obviously won't work. But being totally open and honest with your community about the situation will yield success (given that you have a dedicated memberbase). Just elaborate as to 1) what the financial requirements of maintaining the site are 2) what your current status with the site is (in terms of finances) and 3) how your members can help. Just be honest, and they will know.
And for reference (in which you may be interested), I consider this to be one of the best-put donation drive announcements I've ever seen - very honest and open: Talk Budgies 1st Annual Donation Drive - Talk Budgies Forums. It worked for them in a matter of a few days ($750).
All VIP features, currently valued at around $5 a month. I like some of the ideas here though; basic hosting would be a pretty sweet perk.