This week, you’ll receive a Heroic Vault of the Incarnates piece of loot from the weekly quest, so choose your Great Vault reward after completing it to ensure your weekly reward isn’t the same slot.

You can pick up the weekly quest from within the Adventure Journal (Shift+J).

Complete 4 Mythic dungeons to receive your reward (item level 402-411 loot piece) and open the Great Vault after that.

This way, you’ll make sure the weekly reward isn’t the same slot as what the weekly event quest gives you.