Hello Im Sourish, I'm a Linux, asterisk, network expert. I can install vpn on your server within 3 - 4 hour. I would install OpenVPN with a web portal on your server. You would be easily able to create and download user configuration and start using it. Ony openvpn client can be used at the user end and corfnfiguration can be imported from server. Please let me know and we can talk further on it.
You can try it with me and if you are satisfied then you can pay me. Looking forward to hear from you soon and hope to be a part of this project.
Thanks
Sourish