puupia: if you set vacation days, when your opponents play the game will immediatelly assume those days and include them in the time left (assuming it's not a Fischer clock game or without vacations).
Also your opponents will be able to see on your profile that you're on vacations and for how long instead of expecting you to move every day.
On certain occasions you can even "waste" less vacation days if you set them than if you let autovacations kick in (not that it matters much - we're at the end of the year and you still have 35 days!)