Hi,
I am writing this message to convince you to have a first chat to talk about your project.
We are experienced in VPN solutions (some feedback can find on freelancer account based on completed projects here) and, for sure, will be able to offer you a fully functional solution designed at highest standards and prepared to be easy to use and scalable.
For VPN, can provide probably any reasonable scenario starting with management server(s), access servers, service management interface, user management, authentication, accounting, multiple VPN protocols (PPTP, L2TP, OpenVPN, SSTP/SSL VPN and even others) automatic billing using multiple payment processors, Windows and Android client app.
Will be glad to offer you a good VPN Comercial solution.