I just did this with my original "landline" number from 1999 and a Virtual Number that I bought so the folks back home could call me. Current T-Mobile customer, so I just had it ported at the store to two new SIM cards.
My landline number ported within 24 hours, then I initiated the transfer to Google Voice almost immediately. Total timespan was April 11 to April 15 to be completely from Vonage to Google Voice.
The Virtual Number took almost a week, but as of this morning when it hit the cellphone, I'm free and clear of Vonage. This was expected after I called TMo's porting department and they read the date that Vonage should release the line.