Strange behavior with Custom UV

I’m attempting to “squeeze” the UV’s in a material to achieve a faked perspective. To do this I’m scaling the X coordinates and using the V coordinates to lerp between the scaled coordinates at the top and unscaled at the bottom.
The result is sort of what I’m looking for, but I don’t want this curved effect:

120804-uv_squeeze_curved_pixel_shader.png

I tried using custom UV’s to force it to use the vertex shader, but the results are very strange:

120805-uv_squeeze_straight_vertex_shader_custom_uv.png

Does anyone have an idea on how to achieve this?

120807-uv_squeeze_straight_desired_effect.png

Shader is attached:
[link text][4]

Thanks!
~Mike D.