The Great Vault is insurance. It was created to offer M+ players choices so it wasn’t just a roll of the dice on that ONE item in their weekly chest.
Raiding’s Great Vault needs to be similar insurance, but that requires a different approach.
I think the raid GV should be fundamentally different. You kill bosses, you get “Vault Points”. You pay Vault Points to unlock boxes in the Great Vault. (Again, this is for RAIDING ONLY.) And the boxes have exponentially increasing unlock prices, so you get a lot more out of unlocked 1 or 2 boxes for 10 weeks than spending the same points to unlock 3 (or even 4, if that’s a thing) in 1 week.
The point of this is that, while you’re extending, you spend those Great Vault points you’ve been accumulating all tier, to keep the loot flow coming in when you’re on extend and aren’t getting points anymore.