Is vMotion configured on the same pNIC as the management interface? As the host you could not connect to was hosting working VMs then it must have been up and running and I suspect that there was an issue with the management interface or management agents.
Not sure why your vCenter has moved. Can you check the logs to see if it was powered on on the other host or migrated by DRS?
What happens when you try to remove the powered off VM from the inventory?