Use Vonage SIP

Do I need to use a modem to make Voicent work with Vonage

The short answer is yes, since the only interface to Vonage is the Vonage router/adapter, or the softphone provided by Vonage.

If Vonage opens up its SIP credential to allow any soft phone to use its server, then Voicent can use Vonage service without a modem. With SIP, Voicent software simply acts as a softphone.