Windows – VPN with NAT on a single nic

What kind of VPN are you using? (windows vpn, ipsec, openvpn) I'm assuming the windows one so.

When you say that clients are able to access the private address of the server you mean if from you NAT subnet of VPN subnet?

Did you check if the routes are applied correctly on client site ("route print" on windows, "netstat -nr" on linux). Did you check firewall rules between those 2 subnets? Check using traceroute ("tracert" in windows) the route of the packets between the client and machine behind the NAT (where does it timeout / hangs?

Your setup is no different than how many physical RRAS servers are set up including my own at a small office. If you're talking an RRAS VPN you're more than likely talking about a PPTP VPN... unless you have an internal CA and want to futz around with IPSec. (Hint: If you want to futz around with IPSec VPNs, don't. Get an SSL VPN appliance if security is on your mind.)

Simply forward TCP port 1723 and IP Protocol 47 (GRE) to your RRAS server and you're good to go. Also note that you need to edit / add a Remote Access Policy to allow incoming connections. If I recall correctly, by default no incoming connections are allowed. For instance, I created a group called "VPN Users" and then created a policy called "Corp VPN Policy" that has policy conditions set to allow connections if any incoming request is from a user account that is in that group (as well as making sure that only MS-CHAPv2 PWD is being used, but I digress...). You must elevate that policy higher than the default policy which will deny everything. All of this is done in the "Routing and Remote Access" portion of "Network Policy and Access Services" within Server Manager.

There is no special configuration needed on the clients. Windows's built in VPN client will work like a charm. I've even used Linux machines and pptpclient to connect to it with smashing success.

Now, after typing all of this I realized that this was all true for my Server 2003 RRAS box and you said you had a Server 2008 machine. Your mileage may vary. =)