Returning to dry skin and vitamin D synthesis, this link offers a rather nice explanation: http://www.cholesterol-and-health.com/Vitamin-D.html. The skin lipid required for vitamin D synthesis is a form of cholesterol. I suspect that applying oils to the skin wouldn't improve ability to make vitamin D3, as these are cholesterol-free.

Just to expand on the omega-3 provided by flax and other plant foods, this is ALA. Many people have almost no ability to convert ALA into EPA and DHA. While the body can, at a pinch, use ALA, it has different physical and chemical properties that may make it less desirable for making cell walls, myelin sheathing for nerves, and structural components of the brain. EPA and DHA are also specifically needed for formation, repair, and functioning of the retina.