Hi:
Yes, you can first try out Google Voice (recommended) with a Google-issued phone number, and then, if you're happy with it, port in your own number. The Google-issued number will become a secondary inbound phone number. If you would like to keep it, you would need to pay a fee to make it permanent. Otherwise, it will be automatically deleted off of your account in 90 days, leaving you with just your ported-in number.
The catch: Google Voice only accepts ports-in from numbers held by mobile carriers. Vonage numbers, like other VoIP numbers, are considered land lines. See this link for instructions on how to get around this limitation:
http://www.obihai.com/porttutorialI recommend using either AT&T GoPhone or T-Mobile prepaid for this procedure; don't use one of the reseller (MVNO) carriers.