If it works as it is, I would leave it alone.
I've had my Vonage adapter behind my DIR-655 for two years now with no problems. I made no special setup adjustments.
My network has some pretty heavy WAN bandwidth users on it, particularly video streaming, up and down, and I've never seen a problem.
Vonage needs about 90kbps, which is tiny compared with most WAN connections.
The only circumstances which might require special setup are if you have a really slow WAN connection, or if you have some application which hammers your WAN connection (like torrents or a busy web site).