vapor barrier for below tub deck, over mineral wool, 1926 stucco

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Hello Everyone,

Background info:
- 1926 painted stucco
-boiler fed hot water radiator heat
- Bathroom was mostly gutted (ceiling skimcoated) no prepping for tub install
- naively put up r15 mineral wool couple weeks ago without immediate plans to cover it
- RH upstairs has been hovering ~30-40%
- i noticed layer of frost upon working on tub flange bracing, I've removed the m.wool and the wood planks are currently drying out
- I will be using kerdi board in the wet area and drywall the rest of the room

I will be holding off reinstalling insulation until the wood is dry and i'm ready to cover it with kerdi board above tub deck. My question is what should i do for the below tub deck area 19" x 32" from a vapor standpoint? I was going to caulk and poly that area slightly above deck and the kerdi board will eventually overlap it and the tub flange. To somewhat isolate stud pocket from the main level i installed 2x4" cross members.

Outside of the tub area, still on the exterior wall, i'm working around a window and a 3' cast iron baseboard radiator (drywall still present below window, behind radiator). That wall area is has double faced <1" insulation with furring strips tacked to the face of studs. Should that area be poly'd?

Little bit of a frankenstein on my hands and i don't think air tight is in the cards with the open stud bay from below and working around the radiator.

Any thoughts would be appreciated. thanks!
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