Its been a awhile since I have used vonage, but if i remember correctly its cable modem to router wan port, then depending on the vonage device it will plug into one of the Ethernet ports. The link I provided earlier tells you what ports need to be forward correct for Vonage to work behind a router. Might want to check out the router to make sure all the settings are still correct.
The linksys router you have sounds like the Linksys WRT54G-GL which doesn't have the vonage phone ports built into the router.