1. Yes that is possible
2.
http://www.vbulletin.com/docs/html/
3. Yes, you can do this via the admin cp
4. With template modification you can make vBulletin look any way you want
5. There are no limits on the number of users or sessions, or anything else for that matter. The license just grants you the right to run one public installation on one domain.
http://www.vbulletin.com/order/license_agreement.php