Yes, you can backup vCenter VM with VDR.
Why unfortunately? It is fully supported and even recommended by many community members. Virtual or physical vCenter - it's a matter of taste, both options have strong and weak sides.
MCSA, MCTS, VCP, VMware vExpert '2009
I thing that there is no problem, because you can connect to the VDR plugin using Vcenter user or host user, in case a problem with the Vcenter, you can loging to the host server directly, and restore the backup made with VDR , and start to work.
I hope this info can help you
Are you sure that's possible? The VDR will want to authenticate against the vsphere VM, so I assume you wouldnt be able to restore the Vsphere VM by logging into the V-host directly, if the Vsphere box is down.