Vault was a brand of Pre-War soft drinks produced by The Nuka-Cola corporation in cooperation with Vault-Tec. Vault was no ordinary Pre-War soft drink, as can be expected with anything Vault-Tec creates, it had a more sinister purpose. Being highly addictive and causing acute paranoia after prolonged exposure. It's purpose was to test how a vault with a limitless stock of the sinister potion, full of paranoid schizophrenics would function. (if the state of the vault after it finally opened in 2264 is any indication, then not well.)
Note EditVault was inspired by it's real life counterpart released by Coca-Cola in 2005. 
The slogan for Vault is "Drink Vault, don't be a Do-naught!"