Odd that the difuser cap is inside your apartment.
If the vent only services your apartment, it might work, but if there are any other vents that tie in to a common duct, it could be quite hazardous. Over time, particulates can build up, which might require occasional duct cleaning (and lead to a telltale stain near the external opening).
If you know your neighbors and if they have the same vent in their apt. (that’s not in use). The quick 'Gyver test would be an aerosol-based air fresher and a hair dryer set to high with no heat. And ask if they can smell anything. If so. Then it can’t be used. As stated by @dwardio the fact that the defuser is inside the apt. makes it extra sketchy. I’d start working on a plan B.
I have noted some small particulate on my screen (using magvent to vent through the screen). It has been nothing that a few seconds with rubbing alcohol and a toothbrush didn’t completely clear. I suspect long enough usage, you could theoretically clog a vent, but that would take a VERY long time. Compressed air would probably clear 95%+ of that.
I stayed at a flea-bag hotel one time when I was younger. I asked for a room with a kitchen. Decided to cook up some hamburgers for dinner and turned on the vent over the stove. Next thing I knew, everyone on that floor was out in the hallway complaining about an awful smell in their rooms.
Portable AC unit would pump out hot air - which other apartments wouldn’t want coming in in the summer. However, the fact that there appears to be a “cap” to close it if not hooked up would make me think it may very well be “networked” with the other vents. (How many stacks do you see on the roof?)