Your biggest obstacle will be to drill holes in the vault! There is a bar called Scoot 'N' Blues in Durango, CO that uses a vault for a keg cooler successfully. As to the mushroom towers, most are simple air shaft towers converted to glycol. Unfortunately, temperature conductivity can be poor. Check out the
Kool Rite designed mushroom towers. They are designed specifically for glycol systems and have 100% stainless contact with the beer including the faucets.
Scott Zuhse, Instructor
Micro Matic Dispense Institute