I'm not so sure.
http://www.vbulletin.com/en/license-agreement/

When redirecting, you clearly only use one domain to access the forum. But he specifically asks to use more than one domain to access it.

Anyway, not my problem. Just wanted to make the OP aware that this might be an issue and he might want to clear that use case with vB support through official channels.

I believe for what you want to do, you can use something like this, https://vborg.vbsupport.ru/showthread.php?t=254197

This is an excellent idea to be considered.. and not at all bad for SEO..

check the best implementation here:
https://passport.mobilenations.com (that's 1 single signup/login page) to access all sites visible in header.. I am sure this definitely requires multiple vbulletin licenses.