he would most likely stand outside until you sent someone out to the wasteland. I think you could then bring him in and just let the other person die. lol
FlyingCripple is probably right. Another question will the dwellers not allowed into your vault ever leave?
You have two options. Place a dweller in a production room alone. Keep rushing the room until the fails kill the dweller. Then REMOVE the dead dweller.
Send a dweller out into the wasteland and wait for them to die. Then REMOVE the dead dweller.
When you are at vault capacity, you have to remove a dweller or increase capacity. You cannot increase capacity beyond 200. So if you are at 200, the only way to bring a new dweller into the vault is to remove another from your vault roster. You can only remove dead dwellers.
A bit off topic, but I'm making another vault just for this reason. I've reached the dweller cap on my first vault with 49 rooms constructed and I've noticed a significant increase in chopiness and and sluggishness on my iPhone 6 compared to when I had ..say 80 dwellers. It also became slightly bothersome because whenever I tried to rush a room, it would almost always select a dweller instead. Not sure if it because of the lag or because of my fingers, but it was really annoying when you had to do 20+ rooms.
haha, that happens to me all the time too. I often find dwellers in rooms that are supposed to be empty and thinking, "Where did you come from?" Then spend about 5 mins checking room after room looking to see if there are any dwellers missing in them.