VPN server to access Samba4

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http://serverfault.com – On my network I have an Ubuntu 12.04 server running Samba4, my domain is fully configured and functional. Now, I would like to enable VPN access over the internet, and have another box to do so. I have been searching on the internet for guides and information etc, but have not been successful. I have however found this guide http://www.howtogeek.com/51237/setting-up-a-vpn-pptp-server-on-debian/ but was wondering if I could adapt it somehow to enable access to my DC services. EDIT: I would need to authenticate my VPN server with my DC, if that is possible of course. Any insight would be won (HowTos)