The only vast unexplored area appearing to lead somewhere in the vault is shown in this video. How to access it on the other hand is a different matter. After making the video and further exploration i discovered that the top of the tunnel/entrance is actually the first hole you encounter in the vault next to the floating rock at the end of the video. This can be tested easily by yourself by sending someone into the vault and you watch their gamer tag move around to different areas whilst you wait in the glitched area. Maybe there is a trigger to remove the kill zone so you can access it .... maybe it's just cut content.