Mavericks - VPN command line
On Mac OS X the VPN connections are handled by a daemon vpnd, which should be started at boot by altering /etc/hostconfig so that the line VPNSERVER is set to "-YES-".

Don't do that until you have read and understood the man page for the vpnd configuration plist file and written the file. You can get a good explanation of the file and a template with man vpnd -s 5 at the command line. While testing you can run vpnd from the command line with the "-d" option which stops it going to the background and prints the log info to your terminal session - just the thing for watching it all go wrong.

Have fun! Frankly I pay the $20 or so for the Server app and get the GUI for stuff that fraught with peril - you don't want to get the security on a VPN wrong.

BTW - If you're going to be playing at the command line then here's a tip - you could have learnt most of that by starting with apropos vpn at the prompt.