Basically, altough they don't live in the closet they only visit it.
The name "Vanishing Room/Closest/Shed" is actually a bit of a misnomer, if it were a normal dorm it is where they would go to vanish, except because the area is too small + you claimed the last door into the room they wont actually vanish and will instead immediately go do something else... hopefully constructive. Basically every so often they'll wander to the closet, step into it, then step back out. As if they needed to get something from it.
Note: Sometimes it's hard to target the last myne door, try targetting from the inside of the room instead. Also if a new dormie moves in (ie an old one got.. misplaced), have one of your sims unclaim then reclaim the myne door.
I just downloaded NoUniProtect, too. I wonder how long the dormies will last in the smaller dorm, without a cafeteria...
Until you forget to feed them ...
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Actually since there will only be 2-3 dormies they'll be easy to feed. I've kept a 10-bed dorm fed only serving a couple of meals a day (so some dormies would go hungry sometimes).
Also, if you install a giant pineapple the juice will sustain them indefinitely.