The Shape Layer-setting usually results in a Solid Color Layer with a Vector Mask, but Fill Layers also include Pattern- and Gradient Layers.
John Stanowski wrote:
I just realized that a Shape Layer is the same thing as a Fill Layer which has a Vector Mask....
This is correct, there is no difference, except the default names and the different steps of creation. The Shape layer is a Fill layer with a solid color and a vector mask. If you select a path from the Paths panel and choose Layers > New Fill Layer > Solid Color, you get the same thing just not the same default name.
Iin CS5 and earlier, there isn't much difference because you only have a simple fill and a simple vector mask.
In CS6 there is a more distinct difference, because the shape layer has 2 content generators (fill and stroke) and other properties that a simple vector mask does not have.