Vault 711 is a vault in Gringotts Wizarding Bank that belonged to Sirius Black. It may have been the vault for the House of Black in general, though it is possible that Sirius acquired his own vault after being disowned using the money left to him by his uncle Alphard Black[1].
Sirius used gold from his vault to pay for Harry Potter's Firebolt in 1993[2]. In 1996, when Harry inherited the vault after Sirius's death, it still contained a "reasonable amount of gold"[3]. Given its depth, it is likely a high-security vault.