[booher]

Well, you could *in theory* install 2 copies of vbulletin in the same database with each having a seperate prefix (ex: vb_ and vb2_) then just search and replace all of the calls for the user table in all of the files to call it from the other prefix (ex, if you want installation 1 to load users from installation 2 you would change it to load ONLY THE USER TABLES using vb2_ prefix instead of the vb_ one in the rest of the queries. that SHOULD work... in theory but ey who knows :S lol

Also, be sure to backup database and files before you do that :S lol

EDIT:
looking at the code, you could probably replace all of the:

PHP Code:

" . TABLE_PREFIX . "profilefield 


with:

PHP Code:

PREFIX_profilefield 


and the:

PHP Code:

" . TABLE_PREFIX . "user 


with

PHP Code:

PREFIX_user 


and the same with usergroups, and any other user-related table don't forget to back everything up though in case i'm wrong as i haven't tested this, just theory

just replace PREFIX_ with the prefix of the installation you want to load users from :P and change it on the other one (the one that doesnt use the prefix you want to load users from) and sorry if that's not explained well i suck at explaining. If you have any questsions just ask =) or drop me an e-mail at: booher[at]techsensation[dot]net