The primary difference between P and UP is UP can do unified fabric on all ports, while the P can only do so on the expansion module. So I don't understand why you'd want to mix P and UP. Say you have eth1/20 that you configure as an fcoe port on the UP switch. If vPC failover occurs, eth1/20 which is a fixed port on the P won't be able to do FCOE, so you will have a service disruption.
That being said, I haven't done vPC between a P and a UP so I don't know for sure. But I have a strong feeling it won't be supported for the above reason.
