I like the Treasure Vaults. I've been running them quite a lot in hopes of finding a Promissory Note (or two).
However they still need some slight changes to have the difficulty be perfected. Currently, there isnt much weapon options available to fight the Treasure Mimic. This issue is ever more apparent when you try defeating Tier 2 Mimics with Tier 2 equipment, and even more so in Tire 1 where options are more limited. This can be circumvented very easily by the following changes:

• Tier 1

Completely remove the ability for Treasure Vaults to appear in Tier 1.
There isn't quite much reason for it to be available, as the boss mimic will be too challenging for anyone with Tier 1 gear, and anyone who can fight it will see no benefit in doing them as the rewards won't be worth the effort.

• Tier 2

Remove the tentacles from the fight in Tier 2.
As most weapons in Tier 2 lack adequate enemy penetration capabilities, this makes it extremely hard to fight the boss mimic without a Magnus/Tundrus or Dark Reprisal Mk II.

• Tier 3

Remove some of the tentacles / have the tentacles more spaced out.
This allows for more room for the player to attack the tentacle, and not have weapons be completely intercepted by the tentacles. This would open up more weapon options to bring into vaults, outside of Brandish, Dark Retribution, and Magnuses.

On top of these changes, the slam attack should be given a cooldown, as oftentimes the boss will use it repeatedly making it much harder to deal with than it should be.