Vault-Tec EngineerG 25

Vault-Tec Engineer G25:
Build 50 rooms

For this achievement, you simply have to build 50 rooms in your vault. You will get this achievement naturally as you play, but you can also just build and destroy the same room over and over until you get this achievement to unlock. If you do choose to go this route, the power generator, diner and water treatment plant are the cheapest to build at 100 caps each.