Comcast blast cable, 3.0 modem, Router and Vonage.

I have upgraded to the Blast comcast cable download speeds of 50 if I hard wire to the modem. I have vonage which I ethernet to the modem. Then a linksys E2500 wireless router Etherneted to the vonage box. The vonage will not work behind my router. Which I am working on with Cisco. Only gives me 18 downloads with the vonage in between. I would like to move the vonage to after the router but it doesnt work that way.
Modem only has one ethernet port. Can I buy a network switch to put into the modem and give the router and the vonage their own independent port. I am paying for high speed and want it to work.
I only ask because I spent 4 hours on the phone with Cisco trying to fix the slow speeds and they only slowed it down more. They are calling me back to work on it some more tomorrow.