I am currently trying to understand Vitamin D myself and, for me, the most useful site that I have found is:
http://www.vitamindcouncil.org/
On that site I found these sections to be very useful: #1. How To Get Enough Vitamin D and #2. Understanding Vitamin D Cholecalciferol.

The information is very easy for me to understand and answers many questions that I have had about Vitamin D. So with shorter days approaching and fewer opportunities to get direct sunlight I have some good information on how to supplement my Vitamin D intake.