Is the vapor barrier in the roof vapor-permeable? Yes? No!

Vapor barriers (climate membrane) are more forgiving of processing/construction errors than vapor barriers. Therefore, they should simply be preferred. The additional costs are actually irrelevant, 100-200€. The seminar by the construction manager was probably about cost savings and then the vapor barrier was mentioned.

What I've wondered several times now looking at your photos: why is the bare foil visible at an existing window? I always thought that the foil only withstands UV light for a limited time. Shouldn't there at least be some protection like drywall or something similar in the window area?

My cousin left it open for 6 years. It became so porous that a touch was enough to make a hole in the foil. However, it wasn't a climate membrane but just a standard 08/15 brake.

Most high-quality vapor barriers are UV-resistant.

Cladding around the window is still coming. It doesn't crumble that quickly after all

As already said, the topic has nothing to do with costs.

A standard vapor barrier would be wrong! It has a fixed sd value of 2, which would be too low if anything, and your insulation could get wet.

Bookstar also wrote that a vapor barrier would be wrong and if anything, a climate membrane would make sense.