AFAIK, this just applies to a single running system at a time. Means, you can't run Vista and in Vista run another system in a virtual machine. This would require either origin licence or two licenses of Vista.
Btw, doesn't Vista Ultimate license permits installing multiple copies of Vista on the same device for the same user?
Eula from Vista Ultimate:
6. USE WITH VIRTUALIZATION TECHNOLOGIES. You may use the software installed on the licensed device within a virtual (or otherwise emulated) hardware system on the licensed device. If you do so, you may not play or access content or use applications protected
by any Microsoft digital, information or enterprise rights management technology or other Microsoft rights management services or use BitLocker. We advise against playing or accessing content or using applications protected by other digital, information or
enterprise rights management technology or other rights management services or using full volume disk drive encryption.
So the problem is, we have downgrade right for Vista Ultimate too, can I legally install multiple versions of Windows on downgrade path in VPC using this "extra license usage"?