What are some of the firewall issues relevant to the selection and deployment of a VPN?

"It is critically important that VPNs integrate well with existing firewalls and that the two technologies complement each other. The issues I examine include the perimeter security and whether the firewall will provide the option of adding a VPN to the network, with or without trust. For example, I would configure the VPN to encrypt all the sessions between the firewall and the clients. Anything that isn't encrypted should bump up against and be stopped by the firewall. If I were to connect to the network from an external source, I want it to function as a private connection (encrypted), so it looks and feels like a virtual inside connection, just as if I was sitting in my office."