|“||The vault key is used to open your account at Gringotts.||”|
The Vault Key is obtained by opening an account at Gringotts. It is a required moment in the story, as the vault needs to be opened in order to be awarded 500 Galleons, which are needed to buy all the school equipment.
The Vault Key is used only once, when opening the bank account at Gringotts. Besides that, there is no current known use for the Vault Key in Pottermore.