There is a known bug with sims not showing up for vacation. Usually, when this happens, loading your last saved file and replaying the game to the point of going on vacation fixes it.
However, I get the impression that this is happening consistantly for you. Do you still have an older save that you could go back to?
Sometimes, when a sim, or sim family, is glitched, evicting them from their home and immediately returning them from the clipboard to their home fixes the problem.
However, there is an easier solution which may be worth trying. Select "Edit Town" when you arrive at your vacation destination. Simply go into Edit Town mode - do not save town, and then Return to Game - do not save town. This has worked for me several times.