This happened to me 3 times already, at the same condition. If you get disconnected or if you relog while inside the vault with the boss dead - you will spawn in the middle of the ocean and die. You can’t re-enter the vault because it is closed, and you can’t recover the loot from your body - it’s too far into the fatigue zone.
I suppose this is happening because vaults are physically located under the ocean all around the isle, but still, I’m not sure this is how it is supposed to work.