Running OS X Server, your Airport Extreme configuration will automatically setup (port forwarding) when you turn on the VPN Service in OS X Server as long as you give Server access to the Airport Extreme.
What issues are you having with setting up VPN in OS X Server? If you're having authentication/connectivity issues with OS X Server 3.0.1, then as Linc pointed out there are issues with that service. And the workaround he presented does work. You are on your own for support though if you use it.
Might this explain the issue?
OS X Server (Mavericks): Clients cannot connect to VPN service using L2TP
With OS X Server (Mavericks), the VPN service may no longer accept L2TP connections. This can happen if the client and server are behind different network address translation (>NAT) gateways.
You can configure and use PPTP instead. Please note that PPTP requires Directory accounts.
Thank you all for your help.