Since a few raid tiers it is now possible to determine your weekly Vault outcome for Raid loot by going for specific boss kills. It is (afaik) not quite perfect since clearing the full raid basically broadens those choices again.
Now for Mythic+ you always get (at best) 3 choices out of 40+ with 0 options to narrow those down. Did the devs consider limiting the loot table for the highest ilvl to the key(s) that unlocked this option? Now in case of doing 8 keys above 20+ or rather multiple dungeons that also broadens that again and running Al’gethar Academy(i.e.) 8x a week is quite a lot on the other hand.
The ultimate treat would of course be: Being able to pick from the loot table of the highest key (more choices for having multiple past 20?) and deterministically getting your BIS over a long period of time. With that option nerfs to items or spec interplay would of course be a downside or require a refund option which also needs it’s limitation.
It was my understanding that for raid vault loot once a boss is killed once on each toon that boss’s loot table is permanently open for that toon. I have observed this by receiving loot from LFR Ras after a week I did not kill her for instance.