login unified between vb and external site

[info4me]

Hi all, I need some help to implement an unified login system between vb and my site.
What I like to do is that when a user try to login in my site's login form the code sends a request to vb and verify if the user exists in vb. If user exists it will login both in my site and in forum. If user doesn't exists I need to create in vb.
I found some post talking about this but I have found only a solution to create a new user.
Thanks