Thankfully the number is successfully with google voice. However, am I the only one that finds this experience odd?
How long ago did the number start working at your new carrier?
It's not that uncommon to have some overlap period when parts of both work.
And is Vonage purely VOiP? I think we've seen some reports of virtual numbers continuing to work for quite some time for outbound calls (and possibly forever), as long as the credentials remain in place.
It does seems logical to me that even if that is possible, the old account would close after porting out the number.
Google Voice, just accepted it today.
That I am not sure, but I would assume yes.
Very interesting, and glad I am not alone on this island of thought.
I have similar issue with FreedomPop. I can still call out from FP app 3 months after I ported out my phone number to Sprint. I guess that I could use this feature when I travel overseas and save $0.20/min Sprint would charge me. 
Perhaps that was the 'possibly forever' instance I remembered. I think someone familiar with VOiP indicated it could happen. Convenient as long as it's your number........ could get potentially confusing if you let the cellular version of the number go away.