John.
Cisco has a solution for this, the Cisco Emergency Responder, see the following link:
http://www.cisco.com/en/US/products/sw/voicesw/ps842/index.html
Failing this, I'm not sure there is anything you can do. If you are required to provide 911 emergency service for people using VPNs, then you will need to arrange for local analog access to the PSTN.
You have the option of using Calling Search Spaces of course to prevent them from dialing 911 on their company provided phone, however that may not be something you should do given the nature of 911.
-Adam