When Handsome Jack was killed, the Vault Key shown them the map of all the Vaults! Now, the 9 Vault Hunters went seperate ways, in pairs for the vaults. Gaige is 9th, so she went alone to an undiscovered planet, which was called 3QU15-2, on which TWO Vaults are located. Which she didn't tell them about, they all thought there is only one.
If you notice some mistakes, sorry! I'm not English!