Vmware backup fails immediately with the binary log dump showing:
The ESXi serve hosting the VMs in question is disconnected and inaccessible from the vCenter server.
If the vCenter server cannot access an ESX server and its VMs, these same VMs won't be available for backup and won't be accessible from the VMware plugin.
There could be several reasons why a VM and its ESXi server are disconnected, all outside the NVBU scope.
Please refer to the following VMware KB or contact VMware Support.