Internet and VPN

I have a question about ISP and if they can only serve VPN's

let's say i start a ISP company that only serves the internet through VPN's is that possible, i know a VPN uses the internet but if the internet was a VPN tunnel from my ISP that connect to random DNS servers to connect people to the internet. Is this possible or do you need a internet connection to serve a VPN?

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thanks

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Ricardo