How to Cast Shadows AFTER Vertex Shader/ Updated Geometry?

I have a custom vertex shader on my view model which changes the FOV.

I want the shadows to be cast after the FOV is changed instead of using the original object.

Note how the pistol shadow appears on the projected view model incorrectly.

175654-n.png

Is it a surface shader? You just have to add addshadow to the surface shader directive and it should work:

#pragma surface blah blah blah addshadow