You don't need to put the zone into a different VR. The information in that discussion is for failover. Just specify the host address to the remote gateway in the VR.
set route <IP of VPN gateway>/32 int <backup WAN> gate <next hop>
You would then configure the VPN gateway to use the backup WAN as the outgoing interface.
If you are using a route based VPN, then you would need to configure a tunnel interface and set routes for the internal addresses on the remote site to use the VPN.
set route <internal remote> int tunnel.<number>