Hello
The first thing I will do is below:
Establish a baseline test plan to see what is working before proceeding
Nexus 5K
1) feature vpc
2) Create a VPC Domain between the 5K's
3) Establish the VPC-Keepalive, use the mgmt interface
4) Once the Keepalive has been established, then proceed with the Peer_link configuration
To play it safe you can run an additonal port channel between the 5ks for Non_VPC Vlan's only, this port will be strictly for STP VLAN's. The purpose of this is connection is to make sure you have traffic going between the 5k's, keep in mind that the vpc does not send data between the peer link.
Once you do this you can take the down stream switches and daul connect them to the 5k's and start moving vlans to the VPC Peer link one by one and test to make sure you are still able to connect whatever it is you are testing.
HTH