Inside of your admin console under VBulletin Options (on the left) there is a menu item called Search Type (which links to
http://yoursite.com/admincp/options.php?do=searchtype)
It will tell you the type of search option you have selected. I, for example, am using the VBulletin Search Engine so mine reads:
"Your forum is currently using the vBulletin search engine.
To enable the MySQL Fulltext search, we will issue the following two queries. If for some reason this process is unsuccessful, you will need to perform these queries manually. Please be patient as this can take from 10 seconds to an hour depending on the number of posts on your forum. "
I had the problem of no results. I switched to vBulletin search and then rebuilt my search index (
http://yoursite.com/admincp/misc.php?do=chooser) and everything is working great now.
Hope this helps!