Ominous Vaults, when placed by the player instead of generated in a Trial Chamber, are unlocked using Trial Keys and use the loot tables of normal Vaults.

To recreate:

1. Create a new world with commands and Update 1.21 Experiments enabled
2. Place a Vault block and use the Debug Stick to set "ominous" to true or use the following command: 

   /setblock x y z minecraft:vault[ominous=true]

3. Use a Trial Key to unlock it
4. Compare the results to the loot tables of ominous/trial vaults

Image 1: Loot from various Vault blocks
Image 2: Loot from various Ominous Vault blocks generated inside a Trial Chamber
Image 3: Loot from several manually created Ominous Vaults