There is only one vault symbol in Sub-Level 13, which can be accessed from Titan Industrial Facility, and there’s an optional mission from Davis Pickle that will have you coming here. Whenever you first enter Sub-Level 13, you will have to take an elevator down to go further. As soon as it reaches the bottom, get off the elevator and press the button to send it back up. The vault symbol is on the ground where the elevator was located.

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