Aaron, right now it's set to limit search words to four letters since three-letter searches would add a LOT of extra words to the search database (most of them words that wouldn't likely be searched for), and that, in turn, could put a high load on the servers, which would slow everything down. I don't have specific numbers to know how much it would affect things, but the vBulletin documentation strongly recommends against allowing searches for words shorter than 4 letters.
I know there's a way to use Google to search a specific forum. I'll see if I can find more details about it.