It's not just that. To actually do the interconnection, Vonage would need to build some infrastructure: they'd need trunks into every LATA in which they offered 911. A responsible telco would, it seems to me, see this as an unavoidable part of the cost of offering a service for emergencies reachable by dialing 9-1-1, because you basically can't give callers the same user interaction they expect in an emergency if you *don't* have those trunks. Packet8 is an example of a VoIP telco that is responsible and does the right thing: they *don't* offer a service that works in an unexpected and dangerous way when the user dials 9-1-1; they *do* offer genuine 911 service -- enhanced 911, even, though I have to question whether this is desirable, because of the low quality of the location information for VoIP -- everywhere they have managed to run trunks to. They charge their users separately for this service; we can argue about whether it should be a mandated service or not but it certainly seems equitable to charge for it if it is not mandated, and that's what they do.
What Vonage does is provice fake 911 service that may be adequate for many callers much of the time, but emergencies by defintion are not "many/much" situations: they are emergencies; they are exceptions. And for callers in some areas, the service is worse than inadequate, it actually makes emergencies worse, by wasting the caller's and the emergency personnel's time by sending the calls somewhere that cannot handle them at all.
Thor Lancelot Simon firstname.lastname@example.org
"The inconsistency is startling, though admittedly, if consistency is to be abandoned or transcended, there is no problem." - Noam Chomsky