I notice that a lot of people put their vacation flag up but leave an old, out-of-date vacation message in the profile.
The suggestion is to add a reminder when booking the vacation-- "OK, you've booked a vacation on such-and-such days. Your vacation message currently reads (whatever). If you'd like to change it, click here."
(In fact, it seems that the current message disappears from the left-hand panel after a vacation is booked. Not sure if that's supposed to happen, but even if not, it's easy to miss over there.)