Dr. Michael Holick is one of the major vitamin D experts out there. In this video, he discusses vitamin D relating to bone and muscle health and the prevention of autoimmune and chronic diseases. The reference ranges he uses for vitamin D deficiency are 20 ng/ml. The reality is that anything below 50 ng/ml is low, so the actual statistics are actually MUCH worse than what he says.
He also recommends vitamin D2, and every other expert recommends to NOT use vitamin D2. It is crucial to ONLY use vitamin D3, NOT vitamin D2, as D2 is unnatural and not made in humans normally. Plus, vitamin D3 is less expensive than D2.
He does show that vitamin D doses less than 1000 units per day will NOT increase your vitamin D levels to normal.