"Tip of the iceberg" <- I hope so!
We do have vault-tec:
Having a small settlement deeply burried in a mountain, like a vault. Slaverunners homebase .
Also it would be easier to defend it against raider attacks, due the one, two or three entrances. Those can be easily defended (thinking of a canyon like entrance).
Also a helipad could be added here for the vertibird.
Next idea would be an underwater city. Plainly build into a sea, accessible via a sewer network; an old ship; research rig or something like that.
Needs to be defended from raiders, wild ghouls and mirelurks/lakelurks.
But I don't think the iceberg will be this big .