You just need more sunlight. At tropical latitudes you get enough D in five minutes with only your face exposed. At northern latitudes it just takes longer, or you can expose more skin.
>>sliver+(OP)
If the sun is very low in the sky, no UVB radiation will reach you. The body needs the UVB radiation in order to produce vitamin D. I think a general rule of thumb is that if your shadow is longer than you are, then there is no UVB.