See this thread:
https://forums.vonage.com/showthread.php?t=3604 Essentially it seems to boil down to:
"A Virtual number can be ported out the same as a regular number."
Your circumstances might be different from the poster that received the above response which may lead to a different answer.
Another caveat might be that GV might not accept the number if it's in a rate center they don't support. Hopefully that's not the case since you've ported one already. I just mention this in case this virtual number is in another rate center (in another town?). I guess you may be getting more use out of that prepaid cell for this port.
Hopefully Cindy (at Vonage) has not been downsized and can be reached through their forum if you have any trouble.