Died to a vault with monster at +20 level

Learned a lesson today. A vault in a tier 1 dungeon may contain level 29 monsters.

Alawys open last

The best idea is to clear level and open vault as last thing. If it's too hard just pop movement infusion, or simply run FAST.
Always good idea to return later for some loot.

Vaults are just annoying. So

Vaults are just annoying. So many of them are so easy to clear, then I run into this one, which is so much more difficult than every other vault. I really don't want to read too much into spoilers, so I am just going to leave them alone from now on.

If you can find a source of

If you can find a source of Tracking or if your class has access to the Divination tree or vimsense or other talents capable of detecting monsters at a distance, you can look at the monsters inside the vault before opening a door and learning only then what sort of enemy you'll be confronting. Vision or item-based mapping (there are at least three helpful items in that vein) won't work on a vault, but telepathy/tracking can.