I know this a zombie thread but a nice incentive might be to allow paid members an ad free experience.
I'm not sure if that's an option with VBulletin though and even if it is it wouldn't affect the top banner ads. ETA (or the bottom banner ads.) Just the inline ones. I'm actually OK with all the ads except for the ones that show as a hyperlink on someones post because I think they're linking something relevant and go, nope, fooled again.