Hi Chucky,
As you already know once vPC are operational, if the peer-keepalive link fails, then everything carries on as before. Both switches will still continue to forward traffic on their vPC member ports.
If you were then unfortunate enough to have a failure of the vPC peer link while the peer-keepalive is down, then you get into the scenario where the vPC member ports on the operational secondary device are taken down. You still have connectivity to downstream devices from the operational primary though, and so unless you have single attached devices on the secondary, you're still OK.
"What if another system failure occurs when this keepalive link is down? A switch reboots or a vPC drops?"
If one of the Nexus 5K switch reboots while the peer-keepalive were down, then the remaining N5K will remain or become operational primary and continue to forward traffic on the vPC member ports. If you lost both Nexus 5K of the same pod at the same time as the Layer-2 switch were down, then depending upon your code version and configuration, you could run into issues when they came back up. In the early days of vPC the peer-keepalive was required to initially establish vPC, but Cisco have addressed this issue from 5.0(2)N2(1) with the auto-recovery feature.
If a vPC drops on one or both of the peers e.g., due to a single link failure or the entire downstream device rebooting, then the ports and vPC becomes operational on both the peer devices once the downstream device is operational again. This is irrespective of the state of the peer-keepalive link.
The Virtual Port Channel Operations guide discusses failure scenarios (and more besides), and the use of the auto-recovery and is worth a read to ensure you fully understand the recovery options for all scenario.
In short, I believe that what you're planning is an acceptable risk.
Regards
