Playing with one other person. We moved all chest out of inner vault door , stayed inside looting gold till we died. Spawned at ship, returned to vault to find outer vault door closed and key to vault gone. I've only done two vaults total and the first one didn't close outer door after death. Does this happen a lot?
Outer vault door closed and key disappeared
The entire island resets when there is no one on it, including the outer vault door, spawned enemies and animals.
In order to make sure this doesn't happen, you need to have at least one crew member on the island at all times. If you're solo, you need to be out of the vault before the inner door closes so that you don't drown.
If for some reason your entire crew dies, as long as one person comes back before the others release and go to the ferry you should be fine.