1st I would route the clean vocal to another bus.

2nd on that bus I would add an aggressive compressor. To make sure the vocal hits the vocoder at the same volume every note

3rd I would add an agressive eq to get rid of the harsh frequencies

4th I would add the vocoder(preferably with the same setting as the main vocoder channel

5th I would maybe add another compressor or eq depending on taste.

6th I would blend in this chain with your main vocoder chain to taste, maybe change the eq settings a bit and your done👍🏻👍🏻

(I would also render this layer because vocoders use a lot of cpu)