Vinyl(the other kind)

Yes, but not much

I asked ASC this same question recently 

you might also put a fake ficus bush/tree behind your speakers

Actually they made a huge difference in my room. Without any blinds it was truly horrible, no focus, no snap, bass was booming. Three quarter shut angled up truly worked wonders.
Does not help having a tiled floor either but large rugs and 3 large squishy sofas helped there as well

I've got oversized patio doors along 80% of one wall where I listen. They're covered with S curve vertical blinds. The blinds are far more dead than the glass. Rotating them open a bit makes a difference as they then kinda suck some sound out of the room. I'd say they're definitely better than bare glass. 

I have exactly that behind my Ohm F5s.

Very practical.  Definitely makes a difference in my case and very adjustable. YMMV case by case.