Fortresses only really work if you have a military force to man them, and if some truly catastrophic event happens, your guards aren’t going to stay with your dumb ass to protect you.
At best, they’re going to just abandon you and go home to their families. At worst, they’re going to steal all of your cool shit before abandoning you and going home to their families.
And even if you did manage to skitter down your little hole and lock your blast door before the cataclysm, there’s bound to be some asshole like me in your neighborhood that will decide to make sure you’re fucked along with everyone else.
I’m pretty sure I would spend my dying days stealing a bulldozer and piling about a hundred tons of rock and debris over your door to ensure you never get out of your little tomb alive.
You’re also going to need some sort of surface air-filtration system to pump in clean air, and you don’t even want to know what kind of foul shit I would pump down your vents. And believe you me, it will be some kind of shit because that same truck that vacuums out porta-potties also pumps the crap right back out again.
So mega-wealthy pricks, please feel free to keep building your own personal little Vault-tec vaults and I’ll keep marking them on my map for when the End-Times come.