Tyvek (at least the sort sold by the acre over on the west side of the Atlantic) is not a vapor barrier. It's an air and water barrier. Water vapor can pass through it. Not all that quickly, but it can. That's by design, so that if moisture does get past an interior vapor barrier (holes, laps, etc.) it has a way to keep on going, instead of just staying and soaking the interior of the wall.
Present building codes here require either an inch clear between the underside of the roof, and the top of the insulation, or require enough vapor-impermeable insulation in contact with the bottom of the roof that the dew point would be inside the thickness of that insulation. In that case, that water vapor won't condense. However, water in the vapor form will pass into whatever it can get to, and if the upper side is sealed, you want the lower side to not have a second vapor barrier, so that eventually everything can dry toward one side.
"Building Science" and "Envelope Specialists" have become very important in architectural practice over here. We architects, having messed things up by tossing more and more insulation at our problems, can't be trusted to get things designed properly, let alone design things so that they can be built.
If you have some time, you might see if Joe Lstiburek has any advice for your situation. I do think that removing the plasterboard, ensuring that the framing up there is dry, filling the cavity with vapor-impermeable (foamed in place, likely) insulation, and then replacing the ceiling, may be the best way forward. Might be a summer project so that the framing can dry out. Get a moisture meter and be sure the wood is in the 12% or so range before you re-insulate.