Vonage is SIP, though they use weird port numbers. They will never tell you your configuration, though, so you would have to unlock your existing Vonage adapter so that you can see the settings. I know with a PAP2v1, PAP2v2, or an RTP it's possible to unlock it and still use the adapter with Vonage. Other adapters may allow you to unlock them, but not in a way that allows you to see your settings so you can unlock them.

With a PAP2v1 you'll have to call Vonage tech support and make up some problem, like frequent call drops or something and say you want to downgrade. Say your friend got a downgrade, and his adapter works great at your house so you think the latest firmware doesn't like your router or something lame. Instructions for all of them are on the bargainshare.com forums or other places accessible with Google.

I'll try my Vonage account with the new Connection Manager later this week and see if it works

Edit: Doesn't appear to. The connection manager says it's connected to Vonage, but calls give a server not found (or something) error. Oh, well.. that's why I'm switching to Vitelity anyway