I don't really mean a vault fallout style. I mean what the word vault usually impliest. A very secure room which holds up the most valuable stuff. Kinda the difference between a storage room and a vault, vaults are bigger, they are much better protected and they have the really valuable stuff inside.
I wouldn't think it would be logical the burrowers just dug into the vault simply because if burrowers could dig through the external walls, they could just dig through the walls and infest the rest of the place. The doors would effectivelly be of no use against them.
Rulling out that they could not have dug to it then the next logical step would be to assume that what what opened up the passage for them was the most recent earthquake.
Now that initial room you see when you open the doors is very same, so small in fact that even the storages on the second floor are incredibly bigger so that wouldn't at very least feel like a vault. More like a secure room before the vault. The vault would be then accessible throuh stairs or a passage and I've always assume that passage to be stairs because there is no colapsed walls around it, it looks perfectly like a continuation of the facility for all purposes (yes, with lovelly burrower eggs all around since they decided they didn't liked the decoration).
Since the next map is the hive however then what I'd logicly assume is that somewhere along the line the earthquake colapsed the passage and opened a new one into a burrowers hive, now the logical next step I'd think here is that either this whole place is indeed a hive but a player would be rewarded by later on having a passage into anotehr colapsed walls directly into the vault or that this is actually the vault, only that the initial part of of, almost with all the walls had colapsed but up ahead the scenary would start to change to that of a vault with valuables. Neither is currently the case but we may yet see that change.
Lastly while yes you are told get away from there cause hell is a nicer place than that. You are also lead to belive there is valuable stuff down there... it's kinda like saying. Look, if you go in there you're probably not going to come back... But if you do somehow managed to survive, then you might have just hit the jackpot.
Its not that i don't understand your reasoning Epeli, I just fundamentaly disagree with it... Which is fine cause I love these nerdy discussions