When you say it's a 21 day tournament, I assume you mean the 'hardcore' tournaments that have zero days of timeout period and 21 days of time bank.
Time controls were meant to establish a set amount of time to complete a game or keep it moving, and it adds an element of 'pressure' to both players. Using the vacation feature removes the time pressure from the equation for the player who uses it, while the player who does not use it still has time pressure influencing their game. Removing the vacation feature levels the playing field as far as time pressure goes.
Regular "non-tournament" games always have a certain amount of 'time out' period to make a move and there are no 'time bank only' type of regular games, so dealing with someone who is using the vacation option does not present any serious problem in regular games, at least as far as I know. You just pay attention to all of your games and move when it's your turn.
When the vacation option was first introduced here it was not allowed to be used in any tournament games. At least I'm pretty sure that was the case then, and I don't know why or when that changed and the vacation feature was allowed.
I agree that the vacation feature should not be allowed at all during tournaments. If nothing else, at least ignore it during any "time bank only" tournaments.
I don't play tournaments but that's my opinion for whatever it's worth.