The easiest solution would likely be to set all your clients to default route to the "internet" router - now assuming that is something more than just a $10 ASDL modem you should be able to enter the static routes on that device so that it forwards the PPTP stuff off to the right router for that.
What make/model etc are the routers ?
I'm no routing expert but assuming the Zyxells are like the Drayteks I've been deploying, when you set up the VPN, you specify the remote LAN details and whether or not it should be a route in the local LAN. You do this on both sides so that both routers known about the other router.
When you add a 3rd site you have a choice of building a star shaped network (all VPNs terminate at a central point) or a mesh shaped network (all routers VPN to every other router). It would depend on your traffic patterns on which to build.
If you build a Mesh, it acts the same as your current setup, each router has static routes to every other router. If you build a star, you can usually specify all internal traffic to go to the hub of the star using a simple route, and then the central VPN server will have the individual routes to each other router.
You need to do a little reading about routing to get it all working perfectly, but in the star network I set up with Site A being the centre.
Site A: 192.168.10.x / 255.255.255.0
Site B: 192.168.11.x / 255.255.255.0
Site C: 192.168.12.x / 255.255.255.0
Each site has the default Internet routes, as well as the following VPN routes.
Site A Routes:
192.168.11.0 / 255.255.255.0 -> VPNB
192.168.12.0 / 255.255.255.0 -> VPNC
Site B Routes:
192.168.0.0 / 255.255.0.0 -> VPNA
Site C Routes:
192.168.0.0 / 255.255.0.0 -> VPNA
Best Answer
I have used ZyWALL a few years ago. It was very simple... The only things required were activating the VPN module, specifying local and remote address ranges, and specifying a key. I used SSH Sentinel to connect to the remote private network.
If you use SSH Sentinel, you may have to click on
Acquire virtual IP(or something like that) to be assigned an IP from the range of the remote private network (due to the settings of said private network).It might be a useless question but can those two machines connect to each other?
Edit - make absolutely sure your subnet mapping is correct - example:
This will map the subnets so the computers can see each other. Of course, you NEED to configure your local computers properly - the network configuration of the remote computer should be done in a way, that if you brought said computer over to the local network, it would be accessible.
So configure your network masks, firewalls and routers on servers and client computers so they can communicate.
ZyWALL configuration is so extremely simple that even a basic user should be able to configure it properly. The rest of the network is usually the problem, not the ZyWALL config.