Yes you can use an external sequencer with the XV........ You might want to consider a hardware sequencer (Roland MC series, Yamaha QY {hope that's right}etc, etc) this way you don't have to have the XV by the computer to sequence stuff......
As far as I know there is no "vocoder" effect on the XV...... Are you trying to sing into a mic and add your singing to an instrument (synth lead maybe??) ??????? the XV does not have a mic input (Yamaha S-80 does, if you care... still don't know if it has a vocoder though)
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John 3:16 For God so loved the world, that He gave His only Son, that whoever believes in Him should not perish, but have everlasting life.