For raids, yes. For M+? Nope. I’d like to see a system that helps both. For M+, killing the same boss 3 times is trivial. You can do that each week, on the same difficulty even, no biggie. As such, with your system, if used for M+ too, would allow someone running keys to unlock loot from this new vault every week, while raiders would need three due to lockouts.
Now, you could say that only one kill counts per week for M+, so they’re on par, but then the system behaves differently between the two, and one needs to be special-cased. It’s not intuitive at all, and a pain in the backside to keep track of.
A currency system is simpler, and easier to tune between raids and m+. We can tweak the unlock requirements, that a boss needs to be killed 3 times instead of one to unlock its table, and then it’s essentially the same as yours, except suitable for M+ and raids aswell.
Both in the original idea, and in my currency system variant, the loot is time gated. With the original idea, you get 1 item every 3 weeks (assuming that the counter resets for all bosses when you loot one; otherwise 1 item every week after the first 3). With the currency idea, the currency acquisition rate can be adjusted so that we get 1 item every once in a while, but we’re unable to acquire enough to deck ourselves out in BiS gear within a month.
I mean, if we have a Valor-like system, where we have a seasonal cap, we can only unlock so many items at a time. Even more, on top of a seasonal cap, it’d be one item per week, even when catching up. So not an instant gearing for those who start late in the patch and farm the currency in one sitting.