When you go to the vNIC template, you will see a drop down menu to select the priority you can assign to the vNIC so I dont think you can actually assign more than one...
In regards to the second part of your question... the concept is the same whether this UCS or a switch, let me put it like this.... I had a customer a while ago, who had priority set to 5 and MTU set to 9000 in the vNIC, but the storage array was sending the reply back to the server on priority 3 which had the priority set to 1500, so packets with the DF bit set where not going through...
Bottom line, IMHO, I think you better match priorities across the packet's path so that it does not get mixed... again, this my very humble opinion and I am not familiar with your network and/or requirements.
HTH,
-Kenny