This exploit requires you to start a game with a brand new vault. Place the rooms you start with, and have the Dwellers occupy them. Complete objectives from there, until you are asked to either "Equip 1 Dweller With a Weapon" or "Sell 1 Weapon or Outfit." Instead of completing either of those objectives (by claiming a reward), which will disable the exploit, complete other missions until both of the aforementioned objectives are listed. From there, you should soon see a mission to "Get 50 Water," or to "Get 50 Food," or to "Level Up a Dweller." You can complete any of these three missions repeatedly, as long as you have not first completed "Equip 1 Dweller With a Weapon" or "Sell 1 Weapon or Outfit" already noted. Completing those repeatable missions is an easy way to quickly amass a pile of lunchboxes and coins.