Thanks, we have made some progress with this since i posted.
It appeares you cant connect to MySQL without installing MyODBC (an ODBC driver provided by the people who make MySQL), and as we dont have administrative rights on the server this isnt an option.
I have instead just put a login form on the ASP site that submits to member.php to log in users, and then the ASP site just checks for the presence of the bbuserid cookie. This works for us luckily, because both sites are on the same domain (different subdomains tho).
It means obviosuly i cant access the MySQL vbulletin database, but this is something that can be worked around for us.
We were going to do as you said PPN, and just use the one MySQL database for everything, but we hit up against this necessity of MyODBC problem.
Thanks for your input tho