This one is for an old version, but it's simple and I believe it should work with the latest version of vbulletin:
https://vborg.vbsupport.ru/showthread.php?t=97469
It's not completely automatically installed, you need to follow the instructions to create some custom user profile fields.
If you try it, you might want to post your results in the mod thread so other people will know if it works with vb4 or not.