I looked at this post the other day and decided that I didn't know enough to answer it.
On reflection, I think these questions are best answered by the manufacturer of the repeater and by Vonage. I suspect that any repeater worth the money would have the configuration options to point it to a specific access point. Otherwise, it might pick up access points from the wireless in the facility next door.
Regarding Vonage, I've never used it, but I have used Ekiga, and there was no need to put it in the DMZ. I did, however, have to open the appropriate ports in my firewall and my router.