Vocoder Help

Hey, have any of you ever successfully used a vocoder in GarageBand? I've got a few different plug-ins and just can't figure them out. If I put the vocoder plugin on one of my "real instrument" tracks with vocals on them, then I see the vocals going into the vocoder, but then I need to trigger the synth (or carrier) to vocode, but of course since it's a real instrument track, my midi keyboard won't trigger the other have of the vocoder plug-in. See what I mean? I'm sure there's a way to do it though.

Thanks for any advice.