http://shorewall.org/netmap.html
is the answer for configure it, but as far as i can see, netmap feature for Shorewall currently cannot be configured on any kind of VPN (IPSec, OpenVPN RoadWarrior, OpenVPN Tunnels).
Yes, I think it could be done with some custom templates for vpn and shorewall. I’ll have a look at it when I find time.
I’d concentrate on openvpn first, see http://shorewall.net/OPENVPN.html#Dupnet
I used to have different VPN networks but if that’s not possible this feature could be useful IMO.