Hi ,
Thanks for invitation .
As you need a free VPN android app , I have seen example app you gave .
1] I will develop app and backend also .
2] And I will complete all details you gave like your PPTP VPN server , Radius authentication , In app payment , admob etc .
Please contact me , I am really interested in developing this VPN android app , I have developed various apps with network connectivity and I am expert in android application development .
Please provide me more details (documents) if you have , We can discuss about budget and time over chat , both are flexible .
Waiting for your message .
Thanks ,
Swapnil Chouhan