I haven't used either of these, but there's this mod
www.vbulletin.org/forum/showthread.php?t=267585 but it seems to require editing a vb file. There's also this mod:
www.vbulletin.org/forum/showthread.php?t=242068 but it looks like it only allows you to change the default. Maybe you can merge the ideas and do what you want. (Or maybe someone will know of another mod).