Use vB login on other site?

[Sven]

So here's what I want to achieve:
Running cncforen.de with roughly 10.000 registered users and, on the same server (hardware) but on a different domain, a fan site dedicated to Star Wars: Empire at War (www.sw-eaw.de).
On this site a newsscript and a shoutbox are running and I would like to use the vB login to make news comments impossible w/o being a registered user on our forums and the shoutbox to format the user's nickname different from guest shouts.
So two scripts outside vB are involved and coding this shouldn't be a problem.

All I need to know is:
Is it possible to include the required php files from vB to get a login on the Star Wars site (remember: it's a different domain but the same server)?
Which files need to be included?
I assume I have to use absolute paths in all the included vB files. If so, can I create a variable that extends the current path with the missing part of the absolute one?
Example: global.php currently reads:
require_once(CWD . '/includes/init.php');
The absolute path would be /var/www/myvbdomain/includes/
(CWD would be empty then anyway if it's part of the path, don't know that though)
So it could read then:
require_once($myvariable.CWD.'/includes/init.php');
or
require_once(CWD.$myvariable.'/includes/init.php'); (if CWD has got nothing to do with the path)

where &myvariable would have the value '/var/www/myvbdomain'

I'd appreciate a lot if someone could tell me: You're right, that should work and tell me which files to include.
But I can also live with an answer like 'Forget about it, that's too hard to do'

Another option would be to read from the DB directly - but where's the code that is doing that for vB and can I adapt it?
I don't need user ranks, post count or whatever, just the usergroup (NOT banned, user awaiting...), the username and the password check.