Interior plaster on the exterior wall is not complete

Just to be safe, I would simply smear it with a good amount of dirt.

Is dirt the new construction foam?

In our colloquial language, everything that is almost spreadable, such as [Putz] and [Mörtel]

these Wedi panels seem to me to be a feasible solution. We have a similar problem in another corner, but manual re-plastering is not 100% possible there. Therefore, re-plastering cannot be the sole solution. The plasterer suggests using a vapor barrier in the boxed-in area. However, it is questionable how well this can be attached to the plaster. If the joint edges of the Wedi panels/plastered wall are properly filled, then we create an airtight layer, right?

Just run an acrylic sealant there, then it’s sealed.

Hi

That will still not be sufficient.
The sealing to the ground floor and to the foil behind the drywall board is still not present.

Olli