OK, done a quick search for you on the vBulletin site. The question has been asked a number of times:
http://www.vbulletin.com/forum/showthread.php?t=134083
http://www.vbulletin.com/forum/showthread.php?t=116583
http://www.vbulletin.com/forum/showthread.php?t=111942
http://www.vbulletin.com/forum/showthread.php?t=96323
But it looks like there's no option in the software. We'd have to hack the code. Shouldn't be too hard to implement, but it could be a PITA for future upgrades etc.
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