Definitely tear out the the fiberglass and trace the water source while it's wet...that's the best time to investigate water leaks, when the evidence is fresh. I would not recommend just adding additional layers of poly to cover up an issue behind the walls. That picture looks like a fair amount of water leaking...not just condesation. Tear out, investigate, repair the water leak source, and then go back to adding the vapor barrier.
Actually tyvek is a moisture barrier and an air barrier if sealed properly.
It lets vapour through.
Like Gore-tex. The rain stays out but you stay dry because it "breathes" (i.e. vapour permeable).
Agree. Tyvek is a one-way moisture barrier. It allows water vapor to pass through it, but not the liquid itself. This allows the water vapor that is created inside the house (like steam from a shower or cooking) to escape, but it does not allow rain to get into the woodwork. Nor does it allow a great deal of air to escape from inside the house.