When you use a debug stick to set 'ominous' to 'true' on a vault block it turns the block to look like an ominous vault but does not change anything else. It needs a regular trial key to open and ominous trial keys do not work, still gives the loot of a regular vault and shows the loot of a regular vault cycling through in the centre of the block.

The image below is some loot from the ominous trial vaults that were made with the debug stick, it did seem better than the regular vault loot but after trying multiple i didn't get a heavy core so i don't know whether I'm incorrect about the loot part.