VPN connection at startup (login screen)

Hello everybody, I'm trying to get a Mountain Lion Server to backup daily to my NAS every night. The server is at night in the login screen and I need a script, which get executed by the backup program, that connects the server trough VPN to the NAS, in order to start the backup. I've been looking a lot on the web but I could not find a solution. Is there any? I'm using PPTP but openVPN would be fine too. I reallyyy hope this is feasible because leaving open the afp port on the NAS is not really a solution. Thank you very much! Mattia