The Silver Vaults of Chancey Lane, London
The is one big vault with 72 little vaults which are all shops. Although I don't know how big.
http://www.thesilvervaults.com/history
Does 'former' large vault qualify?
The former home of the Western Canada HQ of long-defunct Canada Permanent Mortage Company in Winnipeg Canada is now home to a payroll service called TelPay. The vault door is a well-preserved, beautiful specimen of the craftsmanship of the J & J Taylor Toronto Safe Works. The vault occupied a wide strip the full length of the south side on all five floors of the building with the main floor door the only access point. The floors are connected by narrow, steep iron staircases. I have no way of ascertaining the total square footage but this is my first exposure to a five-storey vault. Sadly, a precedessor to TelPay broke open the vault walls on all but the main floor so what's left are a series of four balcony-style levels from which the main floor can be viewed. The spaces are used for file storage, offices and meeting rooms. The building dates back to 1909 and has an attractive red sandstone facade that has been lovingly maintained. Were it's original use not disclosed, the main floor interior could be mistaken for the banking hall of a turn-of-the-(20th)-century main bank branch with all marble walls, columns and counters faithfully preserved.