Lycan,
I think this person is saying his current setup is DSL/Cable modem > Vonage device > D-Link router and he wants to change it to DSL/Cable modem > D-Link router > Vonage device... at least this is the way I read his post. The first method is the way that Vonage asks people to connect their devices, http://www.vonage.com/support.php?article=1287&topic=5
I know from previous experience that these Vonage devices can cause all sorts of issues with home networking, especially packet loss and slow speeds (connecting the Vonage router to the regular switch ports on your router may very well fix this).