Surface shader with VertexLit causes black object

Question

I wrote the simplest possible surface shader:

Shader "SimpleSurf" {
    Properties {
        _MainTex ("Base (RGB)", 2D) = "white" {}
    }
    SubShader {
        Tags {"RenderType"="Opaque" }
        LOD 200

        CGPROGRAM
        #pragma surface surf Lambert

        sampler2D _MainTex;

        struct Input {
            float2 uv_MainTex;
        };

        void surf (Input IN, inout SurfaceOutput o) {
            half4 c = tex2D (_MainTex, IN.uv_MainTex);
            o.Albedo = c.rgb;
            o.Alpha = c.a;
        }
        ENDCG
    } 
    FallBack "Diffuse"
}

When the "Forward" rendering path is enabled the object looks fine!

But when the "Vertex lit" rendering path is enabled, it looks became black:

Why it happened? How can I use surface shaders (or maybe write a Vertex/Fragment shader which will consider spot lights) which do some effects in VertexLit path?

Answer 1

Shader "SimpleSurf" 
{   
Properties 
{
    _MainTex ("Base (RGB)", 2D) = "white" {}
}
SubShader 
{       
    Pass 
    {
        CGPROGRAM
        #pragma vertex vert_img
        #pragma fragment frag

        #include "UnityCG.cginc"

        uniform sampler2D _MainTex;

        float4 frag(v2f_img i) : COLOR 
        {
            return tex2D(_MainTex, i.uv);
        }
        ENDCG
    }
}}

Edit: I misunderstood the question. I've replaced the previous shader with the simplest possible vertex and fragment shader that works in the VertexLit render path. If you want objects to receive per pixel lighting you will need to use Forward or Deferred rendering as per http://docs.unity3d.com/Documentation/Manual/RenderingPaths.html

Answer 2

Try setting the fallback to the vertexlit shader. Fallback "VertexLit" I haven't tested it out my self but from my knowledge of the rendering pipeline, I can assume that it will work. Basically, if (and in this situation it will) pixel based diffuse lighting fails, it will look black as there won't be any lighting to compute, it needs something to fallback to, vertex lit.

This edit allows you to preserve the attributes of surface shading without having to stay in deffered or forward rendering paths.