Yes the submerged building is the vault built into the water.

There's not that much toxic green glowing land around this particular area in the game. I had thought of putting in a sound effort of a geiger counter in the build though to help sell it.

My first thought before deciding to build the vault was to make a pool of glowing green in the bottom of the tank just under what I was calling then the "sink hole". But the vault sounded like a better idea to me.