Interestingly, the word vano, the equivalent of "vain" in Spanish has three distinct meanings.
1) Vain as in someone given to vanity, though it is not that commonly used
2) Vain as in "in vain"
3) The opening in a wall which contains a window or a door. It is not the window or door itself, but the hole in the brickwork. This one is, I think, much closer to the Latin meaning of "empty", as a vano contains nothing but air!