Call to Arms: Vale of Eternal Blossoms

This quest is only completable in the BFA invasion version of Vale/MOP. Which means, if your alts or whatever method you choose to farm kills, if they haven't completed the vale questlines then they will not have access to Zidormi and won't be able to change phases.
Edit: To clarify, its not exactly the Vale questline, its the BFA Invasion questline that involves unlocking Mechagon+Nazjatar+Return of the Black Prince.

To be able to complete this you must be in the BfA version of the Vale from the Visions of N'Zoth patch.

To unlock this you must complete the BfA campaign through getting the Heart of Azeroth and the Nazjatar catchup to get level 50 Heart.

Once done you should be able to pick up the unlock quests for the Black Empire Assaults starting with An Unwelcome Advisor for Alliance and Return of the Black Prince for Horde.
Those will lead to the annoying chain required to get the Legendary cloak. However you only have to complete The Halls of Origination to unlock a partial iteration of BfA versions of Uldum and The Vale of Eternal Blossoms that will give credit for kills, and give access to the Ny'alotha raid. If all you want is to get [item=169614] to buy [item=168852] for rank 3 [azerite-essence=15] (for the 5% move speed for speed sets) you only need to go this far to unlock Uldum and the Vale for the weekly Call to Arms quests. You'll still need to fully finish the cloak chain to fully unlock both zones' WQ and full features and BfA Horrific Visions though.