In hindsight it looks obvious, but for some reason for most of my life I never looked closely enough at the Luke and Vader scenes to really understand how the architectural layout is supposed to work. I assumed that the big wing-like structure Luke eventually ends up at is just something he must have landed on after getting blown out the window earlier.
However, if you look closely at the scene where Luke goes back inside, you'll see that the window he was blown out of was right there 2 meters above him! Meaning that he didn't just manage to grab a railing after a long fall, it means that when Vader steps up to look out the window, he's literally almost within an arm's reach. And this also means that the whole encounter must have taken place within that wing-like structure, which means the freezing chamber must be the spherical thing in the upper part of it (although it looks like it's too small to contain it). Which means that after getting separated from R2 by the door, Luke must have walked through a pretty long tunnel (at least 50m, I'd say) to get there, even though on-screen it happens instantaneously.
[attachment deleted by admin]