Seems a little crazy but you could also use another DAW just for your vocoder. I use Logic 9 almost like ‘hardware’ …to run Orange Vocoder.
Run vocal striaght into Logic 9 on left channel. Run synth into Logic 9 on right channel …then output to virtual channel in my Apollo and patch that into Cubase 9.
There ya go…seems actually easier than cubase quadro stuff ha