I love that idea about an underworld vault....
I would just add something like an underground airfield, secret base in the mountain where a part of the mountain opens to fire some Big Berta or nuclear warhead...also a naval base in ruins hidden behind the rocks...all of these areas would be purely for exploration and scavenging...in my place they all exist - even a maze from WWI holding all the tanks and weapons from that era - it is forbidden to access it due to long term deterioration of explosives that turn into nitroglycerin - if it blows it would take a few cities or more because nobody knows how big the maze is...there is also a six floor underground city (it was called nuclear fortress) built under the main capitol with elevators, trains, storage for tanks etc...
All of these above are realistic...I don't know if you could apply the idea of such structures into the soil of United States (PV13)...in my opinion it would be awesome to explore something like that...
http://en.wikipedia.org/wiki/%C5%BDeljava_Air_Base
I found only this air base in English on Wikipedia...ahh its better than nothing...sorry for not locating others...lot of them are on my language or some other...for some I forgot the name, but I've seen some of them...makes you wanna explore every inch of that place - too bad that military is watching all of these places... Its better to live 3 days as a Lion, then spend the rest of your life as a hyena!