i dont know how adsense will affect pr but according to google its adsense program is completely different then search engine program. i dont its a good idea to get rid of vbulletin search since the adsense search appliance relies on google's index of your site which may not always be up to date, thus depriving your users recent and accurate results.
Google searchboxes aren't at all effective for searching within a forum. You might want to include a Google searchbox at the bottom of each page so that your visitors can search the web (and earn you a bit of money), but it's definately best to keep the vBulletin search feature turned on as well.
-- Jeremy Maddock SEOMix.com - Search Engine Optimization Tips My Blog - Business, tech, and politics from a webmaster's perspective