Darth Hade said:

To tell the truth, I never liked the look of "the cave" in ESB. I didn't like that it looked like some sort of hidden or long buried structure. It should have just been a cave, nothing more.

I see Luke coming into the cave and then being met by fog. After a few moments, Vader appears out of the fog.

That's how I would've done it if I was the director. Vader is merely a manifestation of Luke's mind since obviously he is not there. Luke is surprised/shocked to see Vader, but we all know he shouldn' be since he saw exactly what he wanted to see. Making it look more in line with what I would would call a "dream-like" sequence would have been a better choice for Lucas/Kersh in my opinion.

You acknowledge that Vader is merely a manifestation of Luke's mind, so why can't the "structure" around him be a part of this manifestation, as well? I mean, that's how I've always thought of it. You say that it should be just a cave, nothing more, and I agree completely! The difference, though, is that I think it IS just a cave--a cave that's strong with the dark side of the Force. Given the negative energy swirling within it, maybe its appearance changes depending on who enters it. Think of boggarts from Harry Potter--it's the same general concept.