I have talked with support and been informed that the TeraStation is not capable of physically separating the Backup/Replication traffic from the production data because both LAN ports must be on the same subnet. With that configuration all traffic must go to the same gateway in order to get to the destination subnet, whether that subnet is local or on a VPN.
My intent was to not put Backup/Replication traffic on the same subnet as my response time sensitive traffic.
Buffalo needs to update their marketing data to specify that these NAS devices are not intended for business environments with more than one subnet.
I intentionally chose a NAS with Linux for that kind of flexibility, but was informed that it is not possible in the kernel they are using, or they jsut don't know how to create a static route.
I'll be returning these units to my supplier and purchasing a business class set of NAS devices.