Knife the vault door to start the Easter Egg. The four Ultimis characters will begin talking behind the vault door, and the fuse will spawn in (see below).
Replace the Fuse
The fuse will spawn in one of three locations in the room with the PhD Flopper perk. Interact with it by pressing your action button to pick it up.
1. On the table beside PhD Flopper.
2. On the opposite table, near the stairs.
3. On the floor, beside the grey file cabinet
Once you have picked it up, go back downstairs and interact with the electrical box to the right of the vault door to insert the fuse.
Richtofen will say "now I can see the problem."
Proceed to the next step.