Not sure about Vonage but I am within months of canning all my land lines. I have 3. 1 business - 1 home - 1 Fax and they cost me @$230 a month which is the same as my 3 phone cellular bill. I'd rather have the cellular any day of the week because I can call anywhere for the same fee.
I would love to try Vonage but my cable modem is so up and down I think I'd have outages all the time. I pay about $60 a month for two land lines and another $30 for my wife's cell before I pick up the phone. Don't ask why we need three. I don't have a good answer.