Hi,
If you're using "double sided" vPC i.e., a vPC domain on the N7K pair and another vPC domain on the N5K pair, you should not see any impact on the Nexus 7000 except the physical interface(s) connecting to the Nexus 5548 that's being upgraded will "bounce" when the Nexus 5K reloads.
Assuming you only upgrade one Nexus 5K at a time, there should not be any change in the state of the port-channel interface as the interfaces of the vPC connecting to the second Nexus 5K switch of the pair will keep it operational.
In terms of the Nexus 7000, these will not have to be upgraded at the same time.
EDIT:
One thing to add to this is that it would be worthwhile reading through the Install and Upgrade Guides for the Nexus 5000. For example the section
Upgrade Process for a vPC Topology on the Primary Switch and Upgrade Process for a vPC Topology on the Secondary Switch goes trough the step by step process you should use for each switch of the vPC pair.
Regards