I have Vonage also, and I love it. I think it`s too early to tell if the Verizon ruling is the end of Vonage; for now, they are prevented from bringing on new customers. They are also working around the clock on workarounds to get away from the patent problem.
As for phone number portability, yes, you get to take that with you. That`s federal telecommunications law. It doesn`t always work in reverse - taking a landline number and moving it to someone like Vonage. That`s up to the telcom.
But a Vonage number is portable to another carrier, even another VOIP provider. It may get a little sticky if and when Vonage goes under (and frankly I think they`ll survive anyway), but the FCC will be involved and the transition shouldn`t be difficult. (Good thing it`s not part of the ADA, or everybody would start writing in saying how the handicapped should just get over being deaf..)
Anyway, the Vonage crash, if it even comes, won`t be overnight - the FCC won`t allow that to happen. You`ll know well in advance of any changes, from Vonage directly as well as in the news.
I just found an interesting Vonage forum: www.vonage-forum.com. How they keep from getting sued over trademark infringement, I have no idea, but it`s a site for Vonage customers to talk about what`s going on, etc. Might be a good site to bookmark if things ever got ugly.
So for now, I wouldn`t worry too much about it. Ordering more business cards? That`s a tough one...