I have a project that works like this:
User Selects the VPN Level that they want to buy, User is then presented with a Bitcoin deposit address, once that transaction has at least 1 Confirmation on the block chain then the site will generate a user account that is completely anonymous and the password random over the SoftEther API. The User will then be transferred to a VPN configuration page with all the config files for the different VPN modes
The account is good for either 30 days OR if the user has gone through his allotted quota, once this happens the server changes that users group where the user will be behind a captive portal until they reload the Bitcoin address.
These Protocols are being used and the backend for the entire project has already been setup:
OmniCore (supports Omni, Tether and Bitcoin) - docs are here [login to view URL]
SoftEther - API Docs are here: [login to view URL]@api/deki/pages/22/pdf
The Website is [login to view URL] it is just waiting for the wallet front end.
You will have access to the Founder of Tether and OmniCore to ask whatever questions you need to get this donel