The Vault is totally misunderstood imo …
Great Vault is the new weekly chest in Shadowlands located in Oribos (icy-veins). It allows you to unlock gear from PvP and raiding in addition to Mythic dungeons and select a single piece of gear from up to 9 items each week.
Doing activities for each to unlock access to picking from it
For M+ running lower keys does not matter , what matter is your top 10 keys for the week (which can be the same #) having 10x 15s , and having 1000 x 2s does not affect your top 10 runs 