Permanently sealing vent with drywall

[Curlybandit]

Hello everyone. Sorry for the length but this is a unique question and details are needed to explain.

My home was built in the early '80s to resemble a century-home farmhouse. The second floor has 3' walls with 45* slanted walls that lead up to the 8' ceiling. The original owner had it as a bed a breakfast. The main section for the guests (east-west length of building) was separated from the owner's section (north-south attachment on back of main section). The second floor of the owner's section (her bedroom) was sealed off from the second floor of the main home. I believe that in order to aid air circulation on the second floor owner's bedroom, they built a vent into the slanted wall. The vent itself is simply a space between the slanted wall studs with insulation board at the top, the wood of the studs on the sides and a sheet of vent metal on the bottom. It's not a return vent.

The previous owner to us (second owner) connected to second floor back bedroom to the rest of the second floor by opening the wall into the hallway, thereby negating the need for the wall vent. We discovered that the second owner simply stuffed the vent space with fibreglass insulation and covered it over with the vent cover - not a great solution. I removed the insulation and temporarily covered it with a sheet of plastic. We've found that on windy days the plastic will bulge due to air being pushed up the vent passage. I suspect that this is the reason the previous owner stuffed it to prevent cold air from coming into the room.

I want recover the vent permanently and drywall over it.

What I want to do is fill the space with insulation, install vapour barrier over the opening and drywall over it. My only concern is that there seems to be that connection with outside air which is suggested by the pushing of air up the vent when it's windy out. I worry about the possibility that this method of sealing it will result in mold.

Can you please suggest if this is the correct approach or if this should be done another way?

Thank you.