Forum software has never been good with searching a specific term. Especially vBulletin even with Sphinx integration results can be way off. The main problem is that we urge our members to search before post, but in more than 50% searches they are not finding a solution to their troubles.
I have been vBulletin moderator for over 5 years and this has been our constant struggle. We give users warnings because they are not searching and they get frustrated because they are not finding what they are looking for. We "solved" this problem by offering two options to search. One is the regular vBulletin search with Sphinx integration and other with Google. Google results are remarkable for any forum software plus it gives us extra income from Google Adsense. The only downfall is that Google wont search "right now" threads.
I opened this here because I think it is related to Sitepoint search. Please move if its not in appropriate forum category.