"Only allow users to turn their vacation on when they have no moves left to make"
Erik already tried this and it worked out badly. Imagine a player with 25 open games who suddenly needs to take a business trip. Worse, seven of his opponents are online and constantly adding new moves. A few people have suggested a more workable refinement: a player must make at least one move in all games between vacations.
"the cheater"
Someone who suddenly decides to go on vacation when you have a mate in one is a poor sport, perhaps a loser, but not a cheater. They haven't broken any rules and they will still lose, sooner or later, unless you forfeit the game.
I've just got back from my first vacation and I think that there's a couple of simple changes that could be made to help to deter people from using it as a cheating mechanism. It wouldn't eliminate the problem, but would make it much more difficult to use vacation to stretch your time:
This would force cheaters to take a screen shot or set up the position on a board prior to their oppenent making their move, and would not allow the cheater to see their opponent's move until they turn their vacation back off (after which they'd be forced to move before they could turn it back on).
Thoughts?