SmilinSue.... Looks like she has a couple degrees in science, but none in veterinary science and wrote a book on vitamin D deficiency in people. But a credible source? mmmm.... I'm guessing that one person somewhere published this theory on vitamin D in the oils on a dog's fur and everyone else with links saying similar kind of ran with it without researching it further. If I have time later, I'll see if there is any actual science behind this.
My understanding is, as far as Vitamin D goes, dogs get it through diet, not sunlight.