We have our own system (which is not a forum and not related to vBulletin DB). So after user logs into our system, there will be a link to the vBulletin forums (we use email address to authenticate our system and there is a one to one relation to vBulletin DB with the email address). So, when the user clicks on that link, I should be able to redirect the user to forum with out actually user entering the login details again.
I hope the explanation is clear..