***EDIT*** – I rushed through this post and made a minor mistake on the routing, see below.

I”m a bit late to this thread, but I have the solution.
This is my Logic Pro X setup allowing me to live harmonize with MIDI input like the “Prismizer/Messina”

You want 4 channels:

1. Dry Vocal (DV)
2. Hardtuned Vocal (HV) 
3. Hardtuned Rootnote Vocal (RV)
4. OctaVox (OV)

Here’s the setup:

1. DV – Create Audio Track

• Prefader Send to Bus # > creates 2. HV
• ***EDIT*** – Wrong track routing

2. HV – Use pitch correction plugin to hardtune this incoming vocal to the scale of the song you will sing.

• Prefader Send to Bus #+1 > creates 3. RV.     ***EDIT*** – I find this routing creates a cleaner harmony than just a send from DV (especially if you use an insert of Octavox on the main Track Stack to add delays/reverb kind of sounds to the whole vocal harmony chain).

3. RV – Use pitch correction plugin to hardtune incoming vocal to the rootnote of the scale (Single tone only)

4. OV – Create Software Instrument track and choose MIDI Controlled OctaVox instrument.

• Sidechain in OctaVox from RV channel
• Copy settings from the attached OctaVox MIDI Setup picture

You should now have a live MIDI Harmonizer setup (in Logic Pro X)

***EDIT*** – I would also like to clarify this note as saving a Track Stack in Logic does not save the subtracks and routing settings:
*Change Scale and Rootnote settings for pitch correction plugin and Octavox for each Key/Scale and save as Channel Strip Setting per channel to make the process more efficient when setting up to compose or use live*

*It would also be great to have a Chromatic Scale option in the OctaVox settings…*

–Devin