Stainless steel chimney - close vapor diffusion barrier

Dear forum,
we have been living in a prefab house (post-and-beam construction) since 2015. In recent days, we had an exterior stainless steel chimney retrofitted.
For this, a wall breakthrough of about 20-25 cm diameter was made.
A Coolblock from Schräder was installed for the chimney pipe. The chimney pipe runs through this block.
We had to cut through the vapor diffusion barrier for this. Now my question is, how can I close this barrier again?
The barrier is installed about 3 cm deep from the interior wall. First comes the drywall, then a chipboard panel, and then the diffusion barrier.
We would have to close it around the chimney pipe, so to speak. The cutout was clean, meaning the diffusion barrier would first have to be exposed at the edges, i.e., drywall and chipboard would need to be carefully removed to make some of the foil visible in order to possibly glue another piece of diffusion barrier in place with sealing tape.
Afterwards, a round piece of drywall and a round piece of chipboard (in the shape of a donut) would need to be inserted. Here I have the question of how I can insert and glue these two pieces?
Questions upon questions, from the outside the breakthrough is well insulated, but the fabrication inside has to be done on site.
P.S. currently the barrier is open on a 20 cm cutout. Moisture should not be a problem. I assume it must be closed as soon as possible, right?
Best regards

Please upload a photo, so one doesn’t have to strain their imagination so much ;)

Sorry, I am currently on my way to work, attached are the pictures.

To be honest, I don't have a better idea than you. But you are probably not the first to retrofit a fireplace in a timber frame house. I would ask Hausbauer, stove builders, Schräder...

Currently, I would think much more about the sealing on the outside, even if it is clear that it is not finished yet ... But aluminum strips on the chimney pipe and then also sticking them to the plaster does not seem to me to be an optimal or sufficient solution? Wouldn't a (permanently elastic) cartridge sealant (NOT silicone!) around the Coolblock and the plaster have been better? Currently, water is running behind the sealing (aluminum adhesive strips?), first wetting the mineral insulation and then eventually causing the house to rot at that spot gradually. My brother's son-in-law is a master carpenter (in a company that builds timber frame and solid wood houses) who regularly tells exactly such horror stories.

I also strongly advise talking to the construction company to properly seal this spot in terms of possible warranties, both from the outside and the inside.

P.S. What did the chimney sweep say about it? He MUST approve the whole thing ...

Hello everyone,
thank you very much for your contributions.

So, I have made some progress...

-The company I commissioned has an impeccable reputation, and offers for prefabricated houses are not made lightly.
-I googled the silver foil; the material also sounds reliable regarding quality. Below are the product's properties:

Advance offers a wide range of foil adhesive tapes with acrylic adhesives that increase their adhesion over time and, conversely, can be removed if left on for only a short time.
Our tapes have been developed for various applications and guarantee additional performance and resistance to extreme temperatures.
Advantages:
• Offer excellent adaptability
• Are water-repellent and provide a complete vapor barrier
• All Advance foil tapes have a white silicone-coated release liner and unwind very easily
• All tapes from Advance are fully tempered and as soft as the production processes allow.
• All adhesive tapes are easy to tear.

-In image 1 in the attachment, you can see the installation instructions from the company Schräder. On Monday, an interior designer is coming and wants to discuss everything further.

I still have question marks in my head regarding the diffusion barrier. It is still unclear to me how this can be attached to the Coolblock. I assume that the usual adhesive tapes cannot be used. I have inquired with the company Schräder about this. The instructions mention high-temperature sealant. This means some kind of silicone probably needs to be used, and then I am still not sure exactly where (white area) on the Coolblock the diffusion barrier can be attached with adhesive.

I also still have unresolved questions regarding the size of the fire protection board. These things are not exactly pretty, and I assume that at least 70 x 70 cm will be hanging on the living room wall. This raises the question for me whether one should not completely cover the entire wall 2.40m H x 2.20m W with one or two boards and then paint it white...

Maybe one of you has some experience with this.

Regards