I would first check if the bug is caused by your shader, or by something on your PC. Try it with a dummy shader that does nothing, or for instance try it with Slin's shadow collection that uses model 3 and runs fine on my Vista64 PC at work, so it is compatible with Vista. If your problem is caused by some driver issue on your PC than Slin's shadows project wont run on your PC either. If it runs you know that you have to fix your shader.

I know that some shaders run differently on ATI, for instance ATI clips colors to the 0..1 range and nvidia doesnt. But this is a bug of the shader and not of ATI, and certainly not of Vista64.