This applies to whomever is really running Vonage (a company I predict will get bought by private equity in the next 18 months).
This post from Duct Tape Marketing about lifetime value of a customer is dead on the money. If you add up all the former Vonage customers who have left it likely now duplicates what they actually have. If you factor in the word of mouth value of those same customers their referrals likely add up to a large part of the cable MSO's user base.
What's more sad than Vonage losing customers to the MSOs (not that that's a bad thing) is that not one of the existing and surviving VoIP players had the ability to pick up any of the consumer based customers en masse.