Is this just a service selling DNS slaves, or is there something deeper about how this works? From the URL, it just looks like the normal operation of a slave DNS server. What makes it "virtual"?
It's not a slave in that it doesn't get the full zone, nor does it get updates immediately. Instead it does recursion to the original DNS servers, and replies back to the original client. Think of it like a reverse proxy for HTTP, except for DNS.