A good colleague of mine (Michael Biddle) and myself wrote such mod about 2.5 years ago (mod that allowed to have it per usergroup), Modification called "Default Signature Per Usergroup". While back was still vbulletin 3.6.x though. Good chance it's will work as well (if not working, might be very minor changes required to make it work).
You can find Mod here:
https://vborg.vbsupport.ru/showthread.php?t=143158
(and in it's origin here:
http://www.vbhackers.com/f76/default...er-group-5261/)
The mentioned mod above was bit neglected for a while, i guess it will get a 3.8.x version during upcoming weeks.