The vault was placed underwater, based on untextured assets that can be found in the Creation Kit of both an underwater vault window (VltSquidWindow01) and an underwater vault door (VltLGDoorMarine01). Multiple unused scripts for Fallout 4 reference Vault 120. In Fallout 76, there are several unused cells for Vault 120 that cannot be found in Fallout 4's game files. The vault, based on these unused cells, would have included an airlock, a research station and "Stellwagen Gorge."
Vault 120 is also the name of a vault and quest location in Fallout Shelter.
The cells related to Vault 120 only exist in the game files of Fallout 76.