What makes you say that?
EDIT: I could make it a slaver hideout or something along those lines...
........
Totally.... NOT.
The Pitt's strong points are positively elsewhere.
It's got many, very-different, new building facades, all dark and dirty and foreboding, lotsa new exterior structures to play with, and new, specific interior stuff too, and all the scaffolding (its hard to put together btw, it was for me anyway), and new utility walkway stuff.
Adding a vault to it would be like, to me, adding a hot dog stand in the middle of an expensive italian restaurant.
Feel free to do whatever you need to do to learn to use the tool, but, I would hope that as a final thing, you're really using what The Pitt offers instead of doing something like adding a vault to it.
I also note that there are no vaults directly in DC. Vaults were supposed to be protected, somewhat remote places, as opposed with sitting at a relative Ground 0.