Never used a vocoder, and although I DO trigger from the right hand mostly, I do sometimes use LH triggers if I am using an accp pattern. I'll address this more later, but I'm on my way out the door. The biggest advantage of RH triggers to me is in the passing tones and complex chords I can use to really enhance the harmonies. My left hand has always been a bass player for me, and I do not have the dexterity to play really beautiful chords with that hand, so if I'm in arr mode - the chords are kind of simple - that's why I play so many chords on top of the arrangement with the RH.

When I use a harmony patch, the notes are not governed by my hand by the passing tones and subtle chord changes are. I try to think like a vocal ensemble, and phrase accordingly. I'll punch in and out during a phrase, scoop to a new pitch .... lots of smooth, lush effects make the arrangement really stand out, in stead of getting in the way.
My trademark sound is really as a vocal group, not a singer with backups.
TTYL .... gotta get my car inspected!