I don't go on "vacation" but I go to workshops that interest me and work on the breaks.
Contingencies exist! There are other mods who know how everything works, and have been at it a long time. I trust them and they'll do a great job if I get hit by a bus. It would just be hard on them to weather the public side of the job.
The one thing that would suck if I get hit by a bus is that the people who know the code and the people who know how moderation works aren't the same pepole. That's not optimal.
If you go on vacation, is there a backup? And if you retire, or God forbid, get incapacitated from some unforeseen event, is there a continuity plan? I assume contingencies exist