Sorry, you are right of course.
We normally don't provide support for non-licensed members, but you ask a legitimate question.
The answer is no - cookies will NOT work for different domains, even if you frame them. At least this has been my experience. This is because you must specify a cookie domain in the Admin Control Panel, so that cookies will work properly. Only 1 domain can be specified.
You are best to ask this at
www.vbulletin.com for a more definite answer - that's where the vB developers hang out. Ask this question in the Pre-Sales Question there.

Hope this helps a bit at least.