Before we go running off into the Vit D rabbit hole there are a lot of things to look at. (I say this as someone who knew an ICU researcher doing a lot of work of Vit D and ICU outcomes and although it was promising at first it just didn’t hold up in larger, well designed studies. This is a common outcome.)
First, you need to make sure your are comparing the right numbers, case fatality rates vs death rates. The first is based on the number of people positive to Covid vs death rate from Covid vs population. So I don’t think by case fatality rate you can say it was so deadly in the Northeast vs South. The numbers all over the place. The truth is we just don’t have good data yet, that will come later. There are so many variables: is every death classified right, when in the epidemic did an area reach its peak, were the hospitals overwhelmed, what was the age & race breakdown in an area, were there big outbreaks in nursing homes, were there even enough tests. I suspect given what we know of diabetes and high BMI as risk factors the South will have more than enough deaths eventually.
You can see the paltry data we have on case fatality here:
https://coronavirus.jhu.edu/us-map
Also, I KNOW you don’t mean it this way, but talk about Vit D sufficiency or some other “race” related thing is problematic. Race is a societal, not a biological construct. The genetic diversity of people in African is much wider and deeper than other areas, such as people do European descent. There are some traits that can be attracted to people of African background, such as sickle cell. However the population considered “African American” has a lot of European DNA. Some do the highest rates of death are in Latinex communities and some of the Native American Communities are being decimated. I can tell you right now if a researcher goes to a black community ands says “I’d like to test you for Vit D because we think you are dying disproportionately because your skin is dark” the community members are going to roll their eyes and go “Of course it’s the color of our skin to any number of societal issues around race. Would you like to conduct a study on syphilis on our community while you are at it?”