I have successfully written code for both PHP4 and PERL 5 (and they said it couldn't be done...) that tests an entered username and password with that registration files of the installed vbulletin database on the same site. The examples are as follows...
Register at
http://www.ironfist.net/forum/
To test the PHP script goto.. and look on the lower middle section for comments...
http://www.ironfist.net/reviews/view.php?id=1
To test the PERL/CGI script goto.. and look on the lower middle section... enter only a username and password..
http://www.ironfist.net/test/index.cgi
If you're interested you may e-mail me..
jon@ironfist.net or drop me a PM here.. I'm not sure if I'll make it publicly available as yet.