My scenario is:
I have lots of end computers which can directly dial to internet. I have one LAN includes many servers and a ZeroShell.
I want to setup a OpenVPN server on ZeroShell in order to make end computers dial in. Each of the end computers can visit internet with its own internet link, not though ZeroShell, and also can visit servers in the LAN at the same time.
From what I understood from your message in your lan zeroshell doesen’t act as default router/gateway, but as a “vpn appliance” (10.252.252.252 is not the default gateway of your lan). In this case you need to tell to your router (not in zeroshell) that for subnet 10.252.251.0 must use the gateway 10.252.252.252 adding a static route (something like 10.252.251.0/255.255.255.0 gw 10.252.252.252). You can even add this route only in the single pc (the server) if it’s the only one to reach (how to add a route to a windows pc: http://www.howtogeek.com/howto/windows/adding-a-tcpip-route-to-the-windows-routing-table/)
hope it will help, ciao