Sure – you can to 3-way NDMP backup of the data movers Ethernet interfaces on a VG2

It’s actually in the NDMP manual

Technically it is a heterogeneous 3-way backup

Not difference there between regular VNX and gateways

In the NDMP 3-way method, to avoid the backup data traveling on the production LAN, a separate private backup network must be established between all NAS heads and the NAS head connected to the backup device. Metadata and NDMP control data are still transferred across the public network. Figure on the slide depicts NDMP 3-way backup. An NDMP 3-way is useful when backup devices need to be shared among NAS heads. It enables the NAS head to control the backup device and share it with other NAS heads by receiving the backup data through the NDMP.

why are we even discussing 3-way NDMP if you have Avamar NDMP accelerator ? I am sure you can dedicate an available  Ethernet interfaces on your datamover just for NDMP traffic to Avamar accelerator, it does not require FC connectivity.

http://www.emc.com/collateral/software/solution-overview/h7141-avamar-backup-recovery-nas-so.pdf