The vacation feature locks your account, preventing others from attacking it and preventing it from being deleted for being idle. For 12 hours after going on vacation you still may be attacked. This is to prevent the use of Vacation as a means of avoiding retaliation. You are locked out for a minimum of 72 hours (3 days), however vacation can last up to an indefinite amount of time. Vacation ends the first time you log in after the 72 hour period.