"Vitamin D & COVID?"

Vitamin D is important for a number of immunological functions.
So, it does boost your immune system.
Being Vitamin D deficient, is not an ideal state to be in.
Best way to get Vitamin D is sunlight.
If not, you can get the caplets.

People who are melanated, need a little bit more sunlight than those without melanin. A minimum of half an hour a day.

Think of your skin like a solar cell and you get powered from getting exposure to it. Even if it's just sticking your arm out the window or standing by the window and letting that hit you for a little bit. 

It's also great for your mood, for depression and general anxiety.
It helps regulate a number of symptoms in your body.

How much is enough or too much?
You would have to take a test to find out your levels of Vitamin D.