Hi Forrest, and welcome to the forums.
Check the insulation right below the vent for moisture, and look under the insulation for water stains. Also, while you're in the attic look around the underside of the roof for any signs of water stains, indicating a possible leak.
Also, check that the insulation isn't blocking the eves from letting air in. If there is no air entering from the eves then the warm air won't flow up and out the upper vent. However, cold air will flow into the vent, make the ceiling cold, causing mosture from the humidifier to condensate on the ceiling. Just a possibility.
It's possible the peeling has nothing to do with the vent. If the vent leaked at one time and caused the ceiling to need repaired, maybe they didn't prime the new plaster.
I doubt you could tell anything looking from the outside, or at least not withou climbing up on the roof for a close look. But if it's leaking you should be able to tell from inside the attic.
How much of the ceiling is peeling? Is it only right below the vent or spread over the ceiling? I think we could better assess the situation with more detailed information. Take a detailed look at everything that might be related and let us know what you see.