Picked up many good tips from byggahus, but now it's time for my first own post

I've probably been hanging around here for over a year without writing anything myself :eek:
I've renovated an upper floor, and primarily a dormer that was in very poor condition. The entire dormer was torn out and made twice as large so that the toilet and terrace door in the dormer had enough space to function normally.
The house otherwise is old, from somewhere in the 1910-1930s, extended and rebuilt in various stages. No other walls or ceilings on the upper floor where the dormer is located have any moisture barrier (plastic sheeting); what's there is wind barrier paper.
How should I handle the moisture barrier? Do I skip it since it doesn't exist in the rest of the walls or should I install a proper moisture barrier in the walls that are being renovated as I go? I've imagined that there might be a problem with only having plastic sheeting in certain places?
Could I encounter issues if I put up a proper moisture barrier on the drywall in the toilet before it gets tiled (the primer I usually use also acts as a moisture barrier) with regard to the above thoughts on the presence or absence of plastic sheeting?