If you are using multiple VDCs on your Nexus 7000 switch and you create vPC between two of them you can forget about upgrading NX-OS via ISSU. This scenario is not supported, but there is no information about that in documentation. If you try to perform upgrade you will see following error message:

Notifying services about the switchover.            [#                   ]   0% -- FAIL. Return code 0x401E007B (request was aborted by service).
Please issue "show install all failure-reason" to find the cause of the failure.

Failure recovery action::
"Standby will be rebooted to force netboot and image download".
Install has failed. Return code 0x401E007B (request was aborted by service).

Please identify the cause of the failure, and try 'install all' again.

n7k-switch# show install all failure-reason
Service "vpc" in vdc: 2 returned error: configuration lock can not be acquired for peer switch (x41B7004B)
Service "vpc" in vdc: 3 returned error: configuration lock can not be acquired for peer switch (x41B7004B)

The only way to upgrade is to download new software, replace bootvar and reboot whole chassis.