Blizzard changed some of the Great Vault requirements in the latest Patch 10.2 PTR build.
Dungeons Category
The following global strings tied to the dungeon category loot have been changed.
Great Vault Reward Current Level Mythic – The item level of this reward will be based on the lowest of your top 1/4/8 runs this week (currently Mythic).
Great Vault Rewards Current Level Heroic – The item level of this reward will be based on the lowest of your top 1/4/8 runs this week (currently Heroic)
Great Vault Rewards Heroic Improve – Complete 1/4/8 dungeons on Mythic Difficulty to improve your reward.
Great Vault Rewards Mythic Improve – Complete 1/4/8 Mythic Level X or higher dungeons to improve your reward.
How will the item level of loot be determined when you only complete dungeons on Heroic difficulty?
Heroic Dungeons drop loot at a certain item level.
Heroic Dawn of the Infinite drops better loot than standard Heroic Dungeons.
Timewalking Dungeons count as Heroic and drop better loot from Heroic Dungeons.
I assume you will receive better loot when you complete Timewalking Dungeons instead of Heroic.
Here are the Great Vault requirements in Patch 10.2. Please note the Raids category still hasn’t been updated, but should say 2/4/7 Amirdrassil Raid Bosses instead of 3/5/7 Aberrus Raid Bosses.
