I know Vitamin D has come up before around these parts. This is a particularly good summary of where we stand on that right now:
Basically, it comes down to correlation =/= causation - folks may have low D levels for reasons that also make them more susceptible to Covid, for example. Still something to consider for general health and well-being, especially if you’re in one of the groups that tends to have chronically low levels. But it’s not looking anything like a miracle cure.