How Do I Install This External Device?

I have the Digitech Vocalist Live Pro that I want to use to record mock up vocal parts for songs, then replace the Digitech VLP track with real vocals. I love the huge variety of harmonies it has that you can just scroll through. I am unsure how to do this as far as properly connecting everything. I can run my mic cable into the front of the Digitech or should I use the SPDIF and have the Digitech inside Cubase as an instrument? I have no idea where to connect anything. I’m connecting my keyboard to Cubase via USB through my PC, should I also use the MIDI OUT of the keyboard to my Focusrite Pro 40 interface or run that into the Digitech? There are so many options I’m completely confused. I read the manual but it didn’t seem to help. It would be nice to have each harmony vocal recorded to seperate tracks if possible. I should also mention I have an OctoPre MK2 Dynamic connected to my Pro 40 via ADAT for recording drums.

The open ports I have on my Pro 40 are MIDI IN/OUT & SPDIF IN/OUT.