VPN Client for Windows / Mac / IOS / Android

We have a commercial VPN service and need client software written for desktop and mobile devices. The developer will need to have experience of VPN client software and be able to work with our own developers to integrate their software into our linux webserver running Radius authentication and MySQL database. We want to offer OpenVPN / PPTP / L2TP on Windows and Mac and PPTP / L2TP on IOS and Android but are prepared to distribute the work between more than one developer so as to not exclude developers who are not expert in all O/S. We have a rough spreadsheet draft for the software and the options we require so any developer wanting to bid will need to first contact me for a copy of the specification. Bids that have not seen the spec cannot be considered. The format will be relatively simple in appearance and as much as possible, look the same for Windows and Mac. The IOS and Android apps will have the same basic appearance but with the obvious limitations in functionality removed. As mentioned, please message me for further info and a copy of the draft proposal and please give details of any relevant experience that you have had with VPN client software.