[SaintDog]

Last I recall (and from previous instances) you can install one instance per domain, if you require additional instances of the software, you require additional licenses (as per the license agreement).

If the software is setup to appear as if you are running multiple instances, that qualifies for such and you would need multiple licenses.

If you setup vBulletin on a single domain with a single instance and then install the blogging system to blog.domain.com, that would not require an additional license, however use of multiple domains would require an additional license. For instance, if you used domain.com and simply redirected blogdomain.com to domain.com.

There have been quite a few questions regarding the license at the vBulletin support forums and elsewhere and the answer has been the same.

If in question, it would be best to contact Jelsoft directly.