Why not just use the vonage app on a cell phone with wifi? I assume you are going to have internet or wifi? Isn't there that vonage dongle still, plugs into a computer or laptop, enables full access to your vonage account and numbers but remotely? If you port your number to google voice, and HAVE the second box at the 2nd location then you could just put the two numbers into the google voice setup for multi-ring. The quality of google voice from that perspective would be no different than regular POTS service, that's the way it works.

Or, if you don't want to port a primary number to google voice, you could have the 2nd vonage device at the 2nd location, setup google voice, FORWARD your primary number to the google voice number but ring BOTH numbers, or all numbers. Then, just picked whatever line you want wherever you are.

google voice by nature doesn't work over wifi, from a cell phone. The call goes out over the cellular connection, so unlike sype, vonage on the device it doesn't use the wifi connection for the VOIP services.