I recognized that omnious vaults don't drop random loot to a player.

After looting a omnious vault i died and restored ab backup. I got the same loot as before (10 emeralds and an enchanted crossbow). So i started a testing session. I waited different times and did different things before i opened the same vault again. Everytime i restored the backup it was the same loot in the same vault. I tried this with other omnious vaults. Another one gave me 18 Windcharges everytime i resored the backup and opened the vault again.

I saw in the second a new item was shown in the vault and i clicked it with the omnious key, it jumped instant to the item it always drops. 

Unfortunatelly i couldn't test it with another player.