Your "glass smeared with a bit of vaseline" description is how I'd describe my rear window as well. I had assumed it was an issue with the factory treatment because none of my glass is "tinted" and it wouldn't clean up either. But, after seeing this thread I went out to check and my rear glass has a film on it. It has to be a solar heat reducer or anti glare, but, if I pull back the headliner, there is a film edge a millimeter before the glass sealant. Somebody did an impeccable job of hiding the film edges. Not sure about the quarter glass. No film on the door glass or windshield.
I'd give it another go checking for a film.
As to whether a film can be removed without damaging the defrost
I tried de-tinting a rear window once, ended up replacing the glass to fix the defrost.