vBulletin Integration/Login Question

[Pro]

Hey everyone,

I'm currently trying to Integrate vBulletin with another Piece of Software that has a Userbase of 2,000 right now and I was wondering how I could automatically login the users if they were already logged into the other Software?

I was thinking us using an IF statement like:

if($var[4])
{
$not_sure_what['vbulletin_does_to_login_users'];
}

I was trying to go through the vB code but couldn't figure out how to get this accomplished.

Any help would be GREATLY appreciated! Links to somewhere, etc. I tried searching but got nowhere. Thanks!