I work as a dispatcher for the fire dept and we have had some issues with all the VOIP providers, not just Vonage. In most cases we get 911 calls for places nowhere near us (I'm in Ontario, Canada and we have gotten calls from Arizona and Texas). Usually we take the information and try to pass it along but it takes some time, and we tell the caller to hang up and try to call back on a landline or a cell phone. Even if your provider has a so-called 911 service, its a middle man or answering service and you are usually responsible for the extra sign up and providing accurate information. Your call does not go through the 911 system where we automatically get your address and phone number (in most cases). Read the fine print on this subject very carefully. It might be worth still keeping a landline connected in your house for local calls and for emergencies and using the VOIP for long distance stuff.
If you have young kids, or might have a situation where you need help and can't speak (and who can predict that!) I would carefully consider what is important to you, saving $$ or possibly saving your life. The 911 system was designed for a reason and circumventing that system means you may not get the help you need in a timely fashion.
I'm not knocking the VOIP systems, I just don't think they have adequately addressed the problem of not being connected to the 911 system.
Tracey