I did see this in the help section over on Vonage's website... This method will support up to around 4 or 5 phones max - which should be more than enough phones in one home. This method won't work on those living in apartment buildings though...
But still, it is so much easier to get a phone that supports multiple headsets these days.
Yes, I also know how to wire up Vonage using the regular phone wiring in the house. We only need two phones and I love wireless so the two handset wireless phones I have work just right.