I always felt that Vader gave Luke a nudge towards the vents, which then pulled him in once he got close enough (as that initial scene with Luke tumbling towards the vents looks too awkward for him to be naturally sucked in, not to mention it fit with Vader wanting his son to live).

Even so, I always figured Luke was under the "dish" of Cloud City, which brings us to this image:

I think the problem is the perspective of the central shaft, which should be MUCH wider at the top right, considering that Luke would not be that far away from it (the best Luke would have traveled through the tube before being ejected couldn't have been more than a football field length), not to mention that the central shaft would be as wide as a small city.

Also, regardless of flipping or altering the shot of Luke's first entry into the airshafts, should the walls be darker and the lights given any kind of FX to better match it's appearance in the farther perspective shot?:

Okay, it matches better than I first thought, but you get the idea...