Hi,
According to the IEEE 802.3ad Link Aggregation standard, an aggregate link (or port-channel in Cisco terminology) is only allowed between two devices e.g., a single switch and a server. This can be used if you want to add additional bandwidth or provide resilience against link failure, but it doesn't provide resilience against a switch failure.
The Cisco vPC (virtual port-channel) is a proprietary mechanism that allows two Cisco Nexus switches to appear to a downstream device as if it's a single switch. This allows the Nexus switches to work as per the IEEE standard, and at the same time adds resilience against a single switch failure.
The vpc command is that which enables this functionality on the port-channel interface of the Nexus switches.
You'll need the command if you're running vPC i.e., the physical links of the aggregate from the downstream device are distributed across both Nexus switches. If you're going to run standard port-channel i.e., all physical links from the downstream device connected to a single Nexus switch, then it's not needed.
There's some other configuration required to run vPC on the Nexus which you should be able to see if you execute the show running vpc command. You'll see feature vpc, vpc domain etc., that must all be configured prior to actually enabling the vpc command on a port-channel interface. If you say this is working then presumably that's already configured.
Regards
