The rest of the world is fascinated by simple but effective "hacks" like wearing a mask. There are a lot of simple hacks that end up being real.

Both masks and Vitamin D are helpful and those at risk or in areas of great spread should consider both. This is similar to masks and face shields offering a multiplier effect. The big difference is that sufficient Vitamin D levels are strongly correlated with numerous metrics of good health including immune, metabolic, and muscle-skeletal system measures.

Do we have an idea of low the death rate of covid is when the vitamin D deficiency is fixed?

Just how much lower it would get cannot be discerned from available data yet. It is very hard to believe from current data that it would go all the way to zero and also very hard to believe that fixing all vitamin D deficiency would not save any lives. The confidence interval range in between is still large, but it is absolutely (past) time to act on the data because it is so unlikely to not help (or even not help a lot). And the data is so far consistent with the possibility that it could help so much that it would have reduced the pandemic to no worse than prior flu seasons had everyone's vitamin D been brought up above 30ng/ml before SARS-CoV-2 arrived.