One thing missing from your description is where the VPN goes to on the Internet. When you say the other box "has a VPN", can I take it that means that it does so in conjunction with another system, out there on the Internet, beyond the ISP? If so, and you know how to set up the routing etc. (installing some router package), you ought be able to make it work - hard to tell from here.
However, I can think of other ways of doing it on the first system:
- install a VPN client on that system
- if the emergency alarm system uses HTTP (WWW) and supports a Proxy connection, then you could tell it to use one of the
free proxies out there. The outgoing IP address, visible to the ISP, would be that of the proxy.
- Or, also if HTTP-based, install a
Tor client. This is also a proxy solution, but the proxy runs on your computer and can route traffic across multiple external proxies (so it's less likely to fail).
I'd be concerned about the amount of maintenance & monitoring any complicated solution might require. The more complex, the greater the chance ot all goes wrong at precisely the wrong time - especially if you have an ISP that thinks it's acceptable to decide what you can and can't do on the Internet.
