There is great evidence that Vitamin D is at least an indicator of something good, but it looks like it is really sunlight we need.
Sunlight is fine for people south of 30N. I'm north of 42N. I could run around naked in the Summer time and still end up deficient in sunlight generated vitamin D. 10 to 15 minutes with your shirt off in San Diego can net you 20,000 IU of converted vitamin D in your skin. Besides, it has been subfreezing for months in Idaho. I'm not going out without my shirt on.