The system mac and root bridge mac addresses are same on both the primary and secondary vPC. How those are elected and related.
sh vpc role
As Amit says, the MAC addresses are assigned from an internal resource on the switch. On the Nexus 7000 the show sprom backplane command will give you the starting MAC and the number available.
VPC Primary switch MAC-address. I think this is assigned by the switch resoruces internally.