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I think the off gassing of the rust proofing is certainly probable. I wonder if you could pull off the door panel and and let it bake in the sun to drive off any absorbed solvents/vehicles from the rust proof and maybe return it to its former shape. If so, and even if you replace the vinyl, a barrier material such as foil placed behind the panel may prevent a re-occurrence. Polyethylene sheet would most likely be too permeable to be effective so I would avoid that.