Hey TrIn@dOr,
I've searched for a vBulletin mod that does this, but there doesn't seem to be one out there right now.
What i did instead was i created a custom page within vBulletin, and this custom page displayed an iFrame which contains the code of a third-party php/sql script that manages financial stuff. I bought the script for $15.00 (
http://www.phparena.net/scripts.php?script=pacheckbook ) but you should be able to get free scripts out there that does the same thing if you look hard enough.
http://www.hotscripts.com is a good place to search.
So until someone comes up with an accounting / financial management mod for VB i recommend creating custom VB pages using third-party scripts.
I haven't tried out the Joomla-vb mod yet, but there might be an accounting/finances script out there that extends from Joomla that you might want to look into.
Good luck!