Thx for the tips, I am getting closer to the solution, I can already login both to my site and the forums from my login script, BUT...
login.php makes a redirect on line 130:
// do redirect
do_login_redirect();
If I comment out this line then it will not redirect, BUT no vB login will take place either...
I just cannot figure out how to modify includes/functions_login.php in order to skip this redirection AND still log in to vB. Or if redirect is necessary, is it possible to redirect to an arbitrary mypage.php instead?