The UV index is a linear scale; each increase in value corresponds to a constant decrease in time to sunburn. Higher values represent a greater risk of sunburn (which is correlated with other health risks) due to UV exposure. An index of 0 corresponds to zero UV radiation, as is essentially the case at night. An index of 10 corresponds roughly to midday summer sunlight with a clear sky when the UV index was originally designed

It doesn't matter if it's winter or summer, you will make no vitamin D sitting in front of a window

And you can't get adequate UVB exposure sitting indoors or in a car. Virtually all commercial and automobile glass blocks UVB rays. As a result, you will not be able to increase your vitamin D levels by sitting in front of a sunny window, though much of the UVA radiation will penetrate the glass and may be harmful.

Sunlight doesn't actually "provide" you with vitamin D. Rather, your body produces vitamin D when skin is exposed to the sun's ultraviolet rays, which trigger vitamin D synthesis.