This would be a question for The Vicidial Group directly. I believe they have built these systems. However: We have found our best setup to be whitelist IP address configuration instead of VPN. VPN creates a bit of a bottleneck and while it is viable, it must be Enterprise level to allow the necessary throughput speed with no loss or jitter. Whereas IP whitelisting (properly done) requires none of this and has zero footprint on quality. As soon as a link is ESTABLISHED, the system considers it authorized and stops checking it. So far, we've had no hacks (except social) with this system. And a social hack will get past VPN very easily just like whitelisting. LOL
I do know it is entirely possible to run a massive facility over VPN, but this requires a hardware based VPN solution with dedicated networking (still public, but excellent quality bandwidth). This generally results in proprietary hardware and a fair amount of money.
BUT: If you do try it, please post your results. If it works it is certainly worth hearing about.