Vonage is trying to keep people from leaving for other less expensive services like Skype and Ooma. They are also trying to protect their profit margins. If you call them up and tell them you are considering switching to a different service, they will give you much much better pricing.
I called to cancel and they said they would give me free voicemail transcription (usually cost me about $5 per month) and lower my monthly fee by $10 per month. When I told them I still wanted to switch, they offered me three months at no charge. Obviously this may not work out the same way for everyone, but it sounds like they have been instructed to basically give away their service to keep people from leaving.
Even if you don’t want to switch, it is worth calling in, talking to the person in charge of canceling your service and asking if they have any pricing that is more competitive.