I've never found the real-time triggering of harmonies by a guitar to be dependable enough for a gig. Maybe with a keyboard, but the guitar produces too many asynchronous notes in a chord, each of which tries to change the harmony.

That's why I like the vocal processors that let you control the harmonies by continuous controller. It takes a while to set up, but once you have it, it always produces harmonies in the right key, and it always comes in on time. Plus you can turn harmonies on and off at will, use more complicated harmonies if needed, all without being distracted by trying to manually affect the harmony.

If your vocal processor supports MIDI control, I think that's the way to go. BIAB can embed the chords needed to control the harmony, and if you move the song to RB, it offers multiple MIDI ports so your vocalist chords don't accidentally play on your synth.

https://www.dropbox.com/sh/ey71h8ep047758h/AAC12jNM0Hz9Q8zsh5iDeqNHa?dl=0