In cases where all of the configured hosts in VM Backup are managed by a vCenter which is not available or not reachable, a restore operation will not be possible since VM Backup will be trying to communicate directly to the vCenter.

To bypass the vCenter, you would need to disassociate the ESXi host from the unavailable vCenter server and re-add the host under the VM Backup as a standalone ESXi host.

The procedure for disassociating an ESXi host from vCenter, is as follows:

1. Login into the ESXi host directly
   
   
2. In the VMware Web UI, right-click host in the VMware Host Client inventory and select "Disconnect from vCenter Server" from the pop-up menu (this procedure might take some time and it's the same for multiple ESXi versions)
   
   
   
   
3. Once the ESXi host is disassociated from vCenter, you can add the host as a standalone restore specific host where one can perform restore operations to that host directly. Find the guide on How to add a restore specific host here.