How big are vaults?

[Dracontius]

Not their size but how much room is inside for storing? Item slots...

[Zalitar]

Go to singleplayer, spawn in a vault. And check :-)

[Ikki]

300ish

[Dracontius]

Nevermind. Went into Single player and spawned one in. It hold 300 item slots. 5 across and 60 down.

Also took a look at it's actual size, I recommend 6 wide and 5 deep as a minimum to have room to walk around it. You can likely fit one in a 5x5 foundations

[Dracontius]

And 3 floors tall so your next floor will be the 4th floor to build seilings etc.