Looks like it is ignoring any word with fewer than 4 letters (e.g. "mic kit" returns nothing), ignoring numbers (e.g. three, four, five, etc.), and ignoring boolean terms (and, or, not) at least. This looks like intentional behavior, potentially undocumented, with some odd design choices. Screenshots attached, the entered terms are visible in the address bar, while the acknowledged terms are visible at the top of the page.
Thanks Rob. We'll file an issue with vBulletin. We do plan on switching over to Google Custom Search again, now that they've crawled through the whole site. Hopefully I'll have this done along with a bunch of other pending issues by the end of July.