How to set up VPN when the LANs are on stand-alone (ie: D-Link) routers
 

So what I have are two switched LANs uplinked to D-Link routers which access the internet via the WAN side of each respective router.

What I start out with is something like this:

192.168.0.37 on LAN1 --> 192.168.0.1 (D-Link DI-704) --> Internet
192.168.1.15 on LAN2 --> 192.168.0.1 (D-Link DI-524) --> Internet

If what I understand of VPN is correct the "joined" network shows up to all systems as a completely different net section (eg: 10.x.x.x) so I guess that would be "LAN3"?

Much of what I read about for OpenS/WAN assumes the Linux endpoints running the tunnel are also routing the local<-->WAN traffic. Since the LANs are already behind routers I'd like to make use of the "VPN passthru" offered by the D-Link routers at each LAN<-->WAN. For this particular setup I haven't seen any appropriate documentation.

Has anyone gotten the above infrastructure working? If so would you be able to explain how to properly configure the two FreeS/WAN machines (192.168.0.37 and 192.168.1.15 above).

Follow-up (PS): I own two domain names I can use for configuration if necessary (so if the routers, say, need gateway names on DNS I can add them to the zone(s) for one or both of my two domain names).

System (192.168.0.37) on LAN1 runs SuSE10 and my system (192.168.1.15) on LAN2 is Debian Sarge (with linex.org upgrades and extras).