If you put that word into a regular forum post, and try to search for it, do you get a result? The wiki search implementation is currently the same as vBulletin's, we just filter the results to only show what's in wiki forums.
It's possible the problem is with the word itself, where one of the bytes is not being counted as an acceptable character by the MySQL FULLTEXT interpreter, or it's possible that this is an issue with one of vBulletin's UTF-8 functions.