Tim, do you mean you used Google Voice from overseas to the US?
Yes, by using GrooveIp with Android phones you can using your google voice account to call US and Canada from overseas. Google voice using gmail call phone feature from web browser is blocked outside US.
Even I have not use Vonage from overseas, here is my understanding. Once you install the free app and verify your account, you can call any phone numbers in US and Canada for free. Calls between Vonage users are free worldwide. They work like Skype calls and you don't need phone numbers for those.
When I am overseas, I am usually not in the 3G/wifi cloud. Getting a free incoming internet phone call can be difficult. Vonage or Skype are more useful because you can turn on the app when you have wifi and others will know you are online and they can ring you. I buy a local simcard to take care of local calls and receive long distance calls.