from my understanding of the whole set-up you can get the application on your iPod just fine, However you will need a headset, a bluetooth jawbone, the apple microphone + earphones or someother AUDIO input device because the iPod does not have a mic. (if you have the newly released iPod touch you will not need a earphone because it has built in speakers now.)
as for the Vayage service you would need that.
but its like a fraction of a cent a minute so its TOTALY worth it verses a phone plan
-good luck my friend