There may be a shorter way, but this works.
First of all, prepare yourself:
tgm
player.additem 000e6b2e 500
player.additem 000BD56F 1
Head to the exterior of the vault:
coc QuincyQuarriesExt
There are ghouls, so you may want to:
killall
Head to the marker underground which takes you inside. There are ghouls trying to break into the vault, kill them and enter.
There are ghouls inside the vault, kill them and follow the instructions to get inside. You'll need to activate a terminal, take a control board from a ghoul, activate a workshop and then use it to delete material blocking your progress.
You will then meet the Overseer.
Kill the Overseer.
You're the Overseer now.