This is a quick reference guide on how to decipher the Payday 2 secret, once you've gotten into the vault under the White House and are relentlessly attacked by demon cloakers. This guide does NOT address how to get into the vault - this is covered extensively by other guides.

NOTE: I did not discover these solutions on my own. I simply aggregated the information online into a single flowchart. I liberally took solutions & images from r-puffky's GitHub site here:
https://github.com/r-pufky/payday2-secret

-- Thanks to blinya for posting a helpful update in the comments.