i deleted this game about 2 months ago after getting bored but I've come back to play in a different way and am gonna by to share my uncommon tips:
1. Don't upgrade to quickly. A level 3 room with give level 3 events meaning harder rad roaches and tougher flames. I just upgrade to level 2 until the room is full of well kitted out dwellers.
2. Use medbays as grinders. Need to put out X fires or rush Y rooms. Build a med bay 3 wide, don't upgrade it! Then rush it. If you fail you get a minor event your dwellers will easily handle. If you pass you get Stimpacks for future failures
I don't see much point in upgrading it. I rush it for my Stimpacks.
3. Delete the first room and replace with a resource producing room. Why? You'll want your toughest dwellers near the door. I have a power plant there for my strongest dwellers so they can react quickly but keep levelling up.
4. Tired of empty rooms getting infested or on fire and spreading? Spend some Caps. Build an elevator shaft straight down. About 7/8 floors. Then build all your bedrooms and storage rooms down there. I never have dwellers in these rooms so when they catch on fire or get infested they can't spread to the rest of my vault.
5. Play slowly. It's hard not to want 200 dwellers but you need costumes and weapons. I'm currently at 30 dwellers and sitting comfortably. I'm no longer baby booming and exploring the wasteland for weapons and armour and storing it in my very low below ground rooms. It also give your dwellers time to level up and increase their specials.
6. Ignore objectives that force you to progress. Get 16 dwellers pregnant? No. Just delete these unless you plan on rushing. I delete room uprgades and build X rooms and have N babies. It's tempting for that lunchbox but having come back I can say those lunch boxes should not be your buzz for playing. Once you have all the best stuff you quickly get bored. Now I'm playing for vault design and long explorations it's more fun.
7. Build how you like but plan for disaster. I build 3, elevator, 2, elevator 3. I have the Center row as just special rooms except top floor as explained above. The outside rooms and resource or radio. I did this so if only 1 dwellers is training in a special the outside resource rooms can come help him in event of fires.
8. Don't leave rooms empty unless they are separated from everything. If that rad roach infestation spreads you can get overwhelmed. If you design well your dwellers will constantly be levelling up no matter where they are. I try to have at least 1 dweller per 2 person room.
9. Have fun