Is it normal to change the recorded vocals mono audiofiles into stereo? Maybe use the mono to stereo plugin? Or should i leave them in mono?
Generally one would not change them, especially since mono audio files don´t contain any stereo information anyway. But if you´ve got good reasons to do so - do it.
You don’t have to change the recorded vocal in any way, just drag it to a stereo track, then you can use stereo effects such as studio chorus.
IMO it really is a good idea to set up your mono input buses to record to a stereo audio track. If for some reason you definitely need it recorded as mono then you should record to a mono track. But, generally speaking, recording to a stereo track is the way to go.
Regards. 
Say what? That is exactly the opposite of what I would suggest for anyone. Ever…
Please explain.
If you record from a mono input bus onto a stereo audio track, it will reord a mono file (although on a stereo track), but allow you to use stereo inserts on the track.
Exactly 
I can’t remember the last time I recorded to a mono track. I’m sure there are some uses to do it but, for me, nada.
Regards
Totally opposite the way I work, pretty much all mono, if I get stereo tracks to mix I convert most to mono tracks.
There a plenty of ways to use stereo fx later on. Nothing wrong with either way of working, do whatever works best for You.
Myself as well. There is no ‘right’ way to do anything. To each their own.
Though, stereo tracks will use twice the space on hard drive…Just saying.
Stereo tracks don´t use any space on the hard drive at all - stereo files do . But as written above, the files recorded the way I described are recorded as mono files.
Interesting. I just learned something new!
Thanks!
Initially that sounded wrong to me, but dang it’s true. 
Could just be me, but I find that 2 mono tracks vs. 1 stereo track seems to give a bit more flexibility when it comes time for some careful panning placements.
Not on vocals, but say for strings or accordions or something.
Sorry but you didn´t
You keep your mono vocal mono so that the dynamic effects you use are mono. In the days of hardware you wouldn´t put your vocal through a stereo compressor. Also if it´s stereo you can´t use any mono to stereo effects.
If you want to use a true stereo effect you would send to a stereo FX channal or Group.
Also it keeps the panning easier.
But hay each to his/her own.
a singer only has one mouth, so putting it in mono makes sense. if you want to add effects and width then thats a different story. but recording vocals in mono makes more sense to me
It’s amazing how many FX rely on stereo. I used to record in mono and the FX never sounded right and it took me a while to realise that with mono tracks that a lot of FX are only hitting the left side…
Now I usually record in stereo and that way I don’t have to think about it.
And in this day and age, I don’t even think about disk space much.
THAT SAID: On my Annual Feature Request List is that Cubase should have some internal wiring (or at least an option) so that stereo FX work as expected on Mono Tracks… ie. they should automatically effect both sides.
Don´t change them. Mono recorded sources should remain mono for the mixing engineer to know what is what.
Interesting topic, I’m new to recording and testing some Cubase 11 Pro presets: the “AM Vocal 4 Male Ambient Country ST” preset creates a stereo track (ST) with mono signal processing plus stereo reverb on top. My stupid question is: how can I place the vocal dry to the center? The reverb is correctly acting on both sides, but the dry signal is obviously just on the left.