Licensing applies to SSMS 2012 and higher versions only.
License is valid
indefinitely for the major (3.x, 4.x, ...) version you purchased it for.
That same license is also valid for one year from date of purchase for any other major version.
See all licensing details in End-user license agreement (EULA)
License Upgrade has lower cost than buying a completely new version and can only be done
from the immediate previous major version. You can upgrade from 2.x to 3.x or from 3.x to 4.x but
not from 2.x to 4.x. Going from 2.x to 4.x requires a full license purchase.
Unlimited VMs mean that you can activate SSMS Tools Pack on any number of
Parallels, Hyper-V, VMWare, Xen or VirtualBox virtual machines.
The VM check can return both false positives and false negatives in about 0.1% of cases.
Please let me know if that happens.