After clearing the Gearhead Vault you can re-enter it as many times as you want without dealing with enemies. Sprint up to the third floor where there is the big tree in the courtyard and the ATM machines just through the doors in the other room. If you interact with the ATM machines you get cash, then simply sprint and leave the vault and re-enter. You can repeat this for infinite cash. If you linger and casually interact with the ATMs, the payouts are $6, $8, $12-- but if you move quickly and interact with the machines as fast as you can, the payouts are $50, $70, $80 per ATM, particularly from the ATMs to the left as you enter the room (it might just be the last 3 ATMs that you interact with).