If baby is wearing something like this, do you put a vest under the long sleeve bodysuit?

No I don't bother. Baby will tell me if he's too cold but wouldn't want him over heating in a car seat with too many layers. You can always put jackets or cardigans over them if it's cold but the weather is still mild where I live and my house is still 20-22 degrees

I have been when it has been cooler. Today for example, as it’s warmer, I wouldn’t have bothered x

I don’t bother. It’s still two layers round their torso like a sleepsuit and a vest so that should be warm enough. If in doubt I add a cardigan or blanket. Easier to add an outer layer which can be taken off easily than have to strip them right down to take one off if they get too hot

No I never do x x

In cooler momths I'd do a short sleeve vest and sub in a t shirt under but not atm x

I would put a long sleeve vest on or a short sleeve very and a cardigan x

I don't bother, if they are cold I would add a layer on top like a cardigan instead x

No because that would make 3 layers of chest covered and then little one would overheat if I added e.g a puffer or thick Sherpa jacket on if we went out.
Slightly cold won't be the worse thing for them, but overheating can cause many issues like breathing and rashes.