Except sometimes. When I switched from Vonage to Lingo about a year ago, before mandatory E-911, I ported my number the normal way and it worked.
I have no idea whether Vonage could have prevented the port if they'd wanted to. In fact, what happened was that Vonage didn't even notice, and once I got Vonage to cancel the account (I got their attention by turning off the credit card number to which the account was charged), they thought the number was still theirs but unassigned. This had the effect that anyone in the world other than Vonage customers could call me, while Vonage customers got an out of service recording. Since I don't get calls from many Vonage customers, I didn't care.
Then about two months ago, Vonage assigned my old number to a new customer, a student at a nearby college, and I got some rather confused calls from her father who was rather bent out of shape that he called the number that his daughter had given him and that showed up on his caller ID box, and got me instead. Fortunately, I had a simple way to fix it: I happen to be acquainted with a one of the members of Vonage's corporate board of directors, and sent him a note which he forwarded to Vonage management, and it was all fixed within a day. If I hadn't had that side channel, I doubt it would ever have been fixed.