Just leave it mono, Andy. Most vocals are done (and worked with) in mono. No reason to complicate that any.

That being said,... let your ears be the judge. Render (export, whatever) one as mono and one as stereo - then switch back and forth and see if you hear any difference. Go with the one you like best. If they sound identical - go with mono.

(I record my vocals on a hardware recorder and transfer them to my DAW once they are done. They come over as stereo - and that is how I work with them. When I get a vocal from Janice (and Bud), they are mono. I don't have any issues with either)