I believe you should be able to use the VPN. As long as the VPN server supports PPTP or L2TP protocols then all you'll need is the server's address and the login/password you set up. For L2TP you'll need the "shared secret" as well, unless the VPN server supports certificates.
Set it up by creating a VPN connection in your "Network" control panel. Name the configuration and enter the address and account info and if the VPN is set up properly it should provide you with an IP address of the private network.
VPN puts you on the local network, so all you'll need is for the VPN server on the private network to have a public IP address, either directly or by port forwarding to the router/gateway. On the VPN client end all you'll need it to ensure the VPN ports are not blocked on your firewall. Airport extreme is just a router like the netgear one, and shouldnt be an issue as long as you set up port forwarding properly.
VPN utilizes TCP ports 1723, as well as 47(for PPTP), and 15(for L2TP). There are others that may be required, but they depend on your specific configuration. Kerberos authentication uses port 88, and HTTPS uses port 443, etc...depeds on your VPN configuration, but most likely the VPN on the device is relatively simple.