For me this is the demi-plane of dread. The poisonous clouds are the "doorway" around the "plane". Those can be a doorway from anywhere, subject to the whims of the malevolent powers of the demiplane (IE the DM).
So when I have Ravenloft "exist" in whichever campaign world I am using it in, such as GH, FR, Wilderlands, and now Erde, I just consider part of Ravenlofts "special uniqueness" is that it exists simultaneously on multiple planes/worlds across the planes of existance. Which keeps Strahd happy, plenty of fresh blood.