Currently, it sounds on the edge of breaking the License Agreement.
Branding it differently - that'll be the thing that makes it on the border line of breaking the LA. You'll want to check with vBulletin.com Pre-Sales on that.
Though, from previous information, I can give you this list, if you wish to continue to set it up the way you want:
- The forums must be CLEARLY part of a main site.
eg: you do not appear to be running multiple instances of vBulletin.
- You must not try to hide other sections of the website from view on different subdomains.