Huge unexplained data transfer - investigating
I have a few suggestions. I had my phone go over my monthly average by only about 5GB and just about threw my phone in a lake. 

The VPN app claiming all the data transferred as its own is a problem, but I think you can get around that with third party data management apps like 3G Watchdog and My Data Manager. Based on my research on several sites this seems to just... kind of... happen to people randomly across many different configurations. For me the Android OS service was reporting huge spikes, but My Data Manager broke that out into a different service that was running. 


In Apps, clear data and cache for Google Play Services, Google Play Store, Google Services Framework, and Google Backup Transport. 
Go to Settings -> Storage, Cached data, reset. Restart your phone.
Until you've figured it out, try to not be on data but wifi instead. That will minimize overages.
If you're feeling proactive, download one of those data management apps, or both. Then wait.


If it never happens again, I bet you'd be happy.