If you use a RDM disk, then the contend is a Linux filesystem.
You can look for a tool like explore2fs to read this disk (better is to do on a SAN snap of this disk), but it's not simple (and probably it will be too slow).
One idea could be have two destinations: one RDM and one NAS, and simple backup the NAS one.
Or, but it is unsupported, install the BE agent in the VDR VA and use the native agent.
Or, still unsupported, export a samba share of the VDR destination.
Andre