Vpn

The VPN service is non-functional in the current release of Mavericks. You can restore it by copying the file /usr/sbin/racoon from a 10.8.5 installation or its backups.

My understanding is that it is only the L2TP VPN which is not functioning under Mavericks and Server 3. PPTP should work just fine.

See this Apple article for documentation.

How to configure Server for PPTP is found here.

Best of luck.

You're correct. PPTP works fine from the links you provided and L2PT is the one not functioning when the client and server are behind different NAT gateways.