I studied the vBulletin support forum and this a common problem. It appears that three letter words cause undue grief in creating and accessing the search indexes on lots of different forums. Some say there's a minimum length search vBulletin option that can be modified, but it still doesn't seem to work.

I recommend to others to use Google search to do three letter searches with the form:

any three letter search word site:forum.cubcrafters.com