I think I know what you mean.
I'll find the thread in a sec.
http://www.vbulletin.com/forum/showt...threadid=21833
But I remember that it uses flat files, which kinda sucks (IMO).
This is exactly how it's done with the other drop-down fields in vBulletin.
I dunno, but for me it looks like the easiest, cleanest way to do this.
I know it requires a bit hacking, but not TOO much of it.
Maybe a hack can be written to do this automatically...