I had a single rad roach kill 10 of my vault dwellers. This has happened twice, and I still don't believe how I can get the same glitch twice that basically whipes out my vault without any chance of recovery.

It happened 3rd level down with a Level three two dorm Diner both times.

I feel your pain. Radroaches nearly decimated my 2nd vault, and I had to repopulate it from a handful of dwellers and one lunchbox Legendary.

Higher level rooms face more difficult threats. Train up the dwellers' corresponding SPECIAL (Agility in this case) in lieu of upgrading the room; train up their Endurance to prepare for upgrading the room.

Apparently I got "invincible molerat" bug once as the last molerat didn't seem to die no matter how many stimpacks I fed to my dwellers. Finally I decided to evacuate the room and that single molerat stayed there for quite some time, I was afraid that it got stuck and I never get rid of it. Eventually it went back to the hole and new bunch of molerats appeared in next room where I was able to kill them all.