softether VPN client is an open source and free, need to modify :
the application ask for user name and password, must check a WP database if they are valid. ( with API )
then if yes, it download the client vpn files and save it in the local disc, so the user can use to connect to any VPN exist in the list.
the application must download the VPN client files depend on package that the user is subscribed in,
i have 3 packages , every one have specified vpn client files. ( you let me define the server and the directory where the files arelocated of remote access for each package.)
if the user not exist or expired, it ask to login to the site , and renew or register.
the app check in my site if have any updates.
modify and compile Softether VPN client application ( the source code is located here: http://www.softether-download.com/files/softether/v4.21-9613-beta-2016.04.24-tree/Source_Code/softether-src-v4.21-9613-beta.zip )