how to give VB users secure access to a second site?

[SteveC]

I have a second site I want my vbulletin users to be
able to securely access. It's running on IIS.

Ideally, once they're logged into vbulletin I'd like
to be able to redirect them to the other site and pass
their userID, which the second site would use to track
their activity. I tried putting a link in a vbulletin
thread, then using http_referer and a redirect (to
insure they got to the IIS site only from vbulletin)
but some browers seem to block http_refer info and I'm
told it's easy to spoof.

So, I'm thinking I may have to require users to log
into the ISS site as well. I'd like to copy the user
name and passwords from the user database, so all
users would be pre-registered and use the same login.
But since the passwords are encrypted, is there a way
for the IIS site to match them? Can the Vbulletin
encryption be used on the IIS site?

If there is any other simpler solution, like having
the IIS site check the vbulletin database to verfity
the user, that would be ideal.

Sorry this is so long, just wanted to explain what it
is I'm trying to do.

THANKS