unity – Simple shader that scales the user interface image from the center

I'm having trouble shading the user interface images From the center of the image.

The code that is linked below correctly scales the image, But the image does not stay centered.

How can I make a stopover from the center?

Link to Shader code – Gist

The part that is ruining me is that the IN.vertex does not seem to be (0,0) to (1,1). It seems to be (0,0) a (xSize, ySize), where sizes are the dimensions set in Unity.