Mixing in mono is a way to detect phasing and balance problems in your mix before the end user hears it.
Mixing vocals in mono on headphones is a hack that I have found that makes me keep the vocals slightly on top of the music.
I have found that if I mix vocals on headphones but in stereo, I tend to mix them a little loud (which is a whole ‘nuther hack).
When I mix on headphones and in mono and think of the vocal as equal and not really louder than the band, the vocal sits in the track.
When you come out of the mono and go back into stereo, usually find that the vocal is slightly lower than it needs to be. I then can just raise it up by 1-2 db. What this will achieve is a consistent vocal to music ratio.
If you are mixing pop or rock where the vocal is not quite as important, you may want to leave the vocal where it is when you come out of mono. If it’s a more lyric driven style such as gospel or country, you’ll probably want to pull it up a little.
Having the vocal consistent to the music will allow you to avoid that endless tail-chase of “Raise the vocal… wait, now it’s too loud”.
I hope this helps you get your vocal just right in the mix.