Converged links for Eth and FCoE is supported but you still need to follow SAN A/B separation and vPC will break that. So there are 2 options:
If 1 10G FCOE is enough for your storage, then keep the current physical connections and remove the vPC configuration. For ethernet traffic, the UCS FI will dynamic to distribute the vNIC between the 2 uplinks. For the FCOE, you will need to choose one of the link for the FCOE.
If 1 10G FCOE is not enough, then this exercise is more intrusive and requires some down time. It requires unconfiguring the vPC connection and recabling. The UCS to N5K needs to be single-homed. From there, configure for eth/FCOE and port-channeling.