Only let OpenVPN connected users access FTP server - on Windows Server

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http://serverfault.com – Windows server 2008 R2 - I have setup OpenVPN, how do I give users connected to it access to an FTP server on the account (but no other users). I am using a hardware firewall not Windows Firewall. Can being on the VPN somehow 'get round' the firewall? (HowTos)