Backing up VDP with VDP or VDR is currently not supported. Technically, it might work, but it is not supported by VMware Support. You could gracefully shutdown the VDP appliance on a regular basis, e.g. once per week, and copy it to another location. However, this is a workaround at best.
Here is a blog article that discusses some of the challenges you may run into if you try to replicate a VDP appliance with vSphere Replication:
Rest assured we are working on some ways to make it much easier (and better than legacy tape solutions) to move backup data offsite with VDP and VDP Advanced. 