We've learned that tier set items can appear in all nine Great Vault slots including Mythic+ and PvP rows. If you want the highest chance of getting a Tier Set piece, you will want to complete all 9 boxes in Patch 9.2! Tier Set OverviewTier Set Bonuses In Patch 9.2, Tier Sets are returning and one of the ways to acquire them in through the Great Vault. A string indicates that all 9 slots in the Great Vault can roll into a tier piece. This means that if you complete all 9 boxes on your Great Vault, you will have a much higher chance of getting a Tier piece!
NEW GREAT_VAULT_CLASS_SET_TUTORIAL Class Set Items may appear as a reward in all nine Great Vault slotsIt doesn't look like this feature is implemented quite yet on the PTR, so it's unclear what the drop chance will be. We'll find out more once this is actually testable on the PTR, but assuming it can replace the item in any box, this may mean it'll be even harder to get a max item level Mythic+ trinket this tier such as Phial of Putrefaction or Inscrutable Quantum Device, as the loot pool will be diluted further with Tazavesh gear and Tier pieces, and you can't use Valor to upgrade to the maximum item level. However, this will likely be good for alts as you'll get a chance of a Tier Set piece from just doing your weekly activities! The only other ways to get Tier Sets in Patch 9.2 are:
Raiding in Sepulcher of the First Ones. The Creation Catalyst which not much has been revealed about it. Read more on wowhead.com