[Dave]

Originally Posted by Dragonsys

Even if the top domain is the same, and they differences are just sub domains?

Correct.

http://www.vbulletin.com/en/license-agreement/

Provided you comply with the term and conditions of this Agreement and have paid the applicable license fees, vBulletin Solutions grants you a revocable, non-exclusive, non-transferable, , worldwide, limited license to install, download and use a single instance of the Software on a single website domain in a manner consistent with its designs and Documentation as further set forth below and not for purposes of resale or re-compilation, re-distribution, re-syndication, time sharing or use for the benefit for any third-party (the "License"). Upon vBulletin Solutions? receipt of the applicable license fees, a unique license number will be issued to you.

1.4 Domain Use. The Software may only be used on a single website domain. Any modification of the Software intended to circumvent the foregoing is prohibited and will result in revocation of the License. Notwithstanding the foregoing, the License may be transferred to another domain with vBulletin Solutions? prior written consent and a transfer fee subject to the vBulletin Solutions then current fees.

A sub-domain is also considered as a single domain.
You may only use the same vBulletin license on a domain that's private and only for testing purposes.