Why not build it into fallout 4 much like was done for Evil Genius. Could be multiple plot devices and missions for getting stuff for your bunker/fallout shelter.
Perhaps have the character find a congressional key card that opens a door to a series of Mr Gutsy construction bots awaiting the customized construction of the congressman's personal shelter but additional materials are needed to complete the project etc.
All sorts of replay ability then can ensue, defend your shelter from all manner of wasteland hordes etc.