Dos and Don’ts
Do: Expand slowly
Building up your vault takes energy and manpower, and if you expand too quickly, you'll rapidly run out of the resources you need to keep the place functional and your dwellers happy. Build out slowly, making sure you can handle your vault at its current size before expanding more, and you'll be on your way to a happy community.
Do: Focus on building power plants before expanding your shelter
Water purification plants, restaurants, and living quarters keep your dwellers happy and healthy, but power plants are what keep the whole thing operational. Without juice to keep them running, none of those other facilities will work, and you'll have a lot of disgruntled residents sitting around in the dark. Make sure there's plenty of electricity to go around before putting even more of a strain on your power plants.
Do: Put critical facilities near power plants
Sometimes, as hard as you try, you may accidentally build too quickly or lose your supplies to raiders you weren't prepared to engage. If that's the case and you dip below the power levels required to keep the vault running, the rooms farthest from your power plant will go down one by one. To make sure your vault is still operational in the event of an outage, put your most critical facilities like restaurants, gardens, water treatment, and medical bays near power plants, so they'll stay operational the longest.
Do: Equip your explorer before going out into the wasteland
It's dangerous out there in the wilds, and an unarmed and underdressed dweller won't be able to defend themselves against all the bloatflies, wild dogs, and radroaches the wasteland will throw at them. Plus, when they're barely able to fend off a giant ant, chances are they won't be nabbing you any good loot. Make sure your explorers are well equipped for the journey before you send them out if you want them to survive to see another outing. A good weapon and high-endurance outfit should do the trick.
Do: Match dwellers to the right room so they’ll be more productive
The dwellers in your vault are a productive bunch who are much happier when they're working a job they love. To that end, instead of assigning jobs at random, look over your dwellers' special stats and see what tasks they're best suited for first before putting them to work (and check the guide at the top of this if you aren't sure what skill matches what job). That'll boost the happiness of your dwellers and increase production efficiency, so everyone wins!
Do: Watch for kids who grow to adulthood so you can put them to work quickly
The folks in the vault apparently have a thing against child labor, so you won't be able to put your whippersnappers to work until they come of age several hours after their birth. They'll also blithely squander their youthful minutes by hovering at 50% happiness throughout their entire childhood, so you'll want to put them to work as soon as possible once they've grown up. If you have so many dwellers that you can't figure out who's just taken their first steps into adulthood, check your dweller roster. Anyone who has the "Coffee Break" job designation is probably the young adult you're looking for.
Don’t: Knock up every woman in your shelter
Aside from the obvious unpleasantness involved in turning your vault into a baby farm, getting every woman pregnant at once also presents practical problems, as it rapidly increases your population without giving you a functional workforce. In the hours it takes those kids to grow to adulthood, the vault's infrastructure could already be crippled, and getting it back on track may be impossible when all those newly-minted adults carry over radiation poisoning and ill health from their childhoods.
Don’t: Rush if your chance of failure is greater than 40%
Rushing is one of the quickest and easiest ways to get extra supplies, but it comes with an extra risk: if it fails, expect a radroach infestation or freak fire, and for the room's production clock to reset. That's a reasonable risk when the chance of failure is less than 20%, and still all right into the 30% range, but over 40% and you are destined for catastrophe at least two out of five times you try. Try rushing different rooms instead, or sending dwellers out into the wasteland to find them.
Don’t: Let your dwellers idle
The less your dwellers work, the sadder they'll be, and the fewer people you'll have contributing to the productivity of the vault. Seek out any dwellers on a coffee break (which you'll find in their job designation) and put them on a useful task, whether that means working a job, exploring the wasteland, or beefing up their stats. Everyone will be glad you did.
Don’t: Leave one dweller to handle a roach infestation
Though they may not seem all that dangerous, radroaches will rapidly decrease the health of any dweller that's sharing a room with them, and that goes double if there are more roaches than people. Leaving one dweller to take care of a whole infestation basically guarantees that there'll be a funeral in the vault's near future, and the vermin will spread to more rooms if they're not dealt with ASAP. So send whoever you can spare to deal with the problem before it gets out of control - preferably someone armed and capable.
Don’t: Worry about leaving the game
It might seem nerve-wracking to leave your vault to the mercy of whatever horrible insect plague might be visited upon it, but there's no need to fear turning the game off. As long as you exit to the main menu before leaving the game, the vault keeps on trucking - but only good things will happen while you're away. Babies will be born and resources will be produced, but raiders won't invade and nothing will catch fire. Rest easy!