Although you can purchase Windows 2012 and use downgrade rights to install Windows 2008, you cannot legally buy Windows 2008 any more, so any quote you have been given about this is wrong.
The Windows 2012 rules have been quoted, but just to add-
If you have a cluster, and your expectation is for a Windows license to fail over in a VMware HA event, you aren't automatically entitled to do this, since this is effectively about the reassigning the license to a different physical CPU on a potentially regular basis. You can either:
Purchase Software Assurance, with includes such rights
Purchase Windows Datacenter Edition for both hosts, then use the new license on the new host. This is far more cost effective.