Whole milk and vitamin D milk are not the same thing, but a lot of the whole milk sold in the United States is fortified with vitamin D.
Whole milk is milk that has a fat content of 3.25 percent. Naturally, the fat sits on top of the milk. However, most milk sold in the United States is homogenized, which means that the fat is emulsified so that it blends into the milk. If a dairy decides to fortify whole milk with vitamin D, the milk must be labeled accordingly and can only contain an 400 International Units of vitamin D for every quart of milk.