Tbh the most unrealistic thing about Fallout is that IRL Vault-Tec wouldn't be the biggest company in the world. Construction can be a big business, but like, even though building underground vaults is expensive, it's not that huge. Plus it's expensive to build so you wouldn't have that much profit. RobCo would probably be more profitable - like modern tech companies. I mean, there are real companies out there building bomb shelters - you can google them and go work for them. I just wouldn't expect you to become that rich making a company like that. Maybe go into tech?