Multiple pass Surface Shader

I am working on a multiple pass surface shader based on the Unity builtin VertexLit shader...

Basically, what I am trying to do is let the shader do its thing in its own passes, and then add a second pass to layer some extra color on top of what color was originally there.

I've gotten that far, however the problem is, my surface shader pass is ignoring transparency -- returning an alpha of zero should cause colors from the the original VertexLit shader to bleed through, right? Apparently not!

I am very new to any form of advanced shaders, and clearly there is something here I don't understand. Does anyone care to give a bit of clarity to a noob? Thankyou!

Here is the code for the aforementioned shader: -- I've noted the part of the code that stores my "secondary pass"

 Shader "VertexLit Toon" {
 Properties {
     _RimPower ("Rim Power", Float) = 2.5
     _Color ("Main Color", Color) = (1,1,1,1)
     _SpecColor ("Spec Color", Color) = (1,1,1,1)
     _Emission ("Emissive Color", Color) = (0,0,0,0)
     _Shininess ("Shininess", Range (0.01, 1)) = 0.7
     _MainTex ("Base (RGB)", 2D) = "white" {}
 }
 
 // 2/3 texture stage GPUs
 SubShader {
     Tags { "RenderType"="Opaque" }
     LOD 100
     
     // Non-lightmapped
     Pass {
         Tags { "LightMode" = "Vertex" }
         
         Material {
             Diffuse [_Color]
             Ambient [_Color]
             Shininess [_Shininess]
             Specular [_SpecColor]
             Emission [_Emission]
         } 
         Lighting On
         SeparateSpecular On
         SetTexture [_MainTex] {
             Combine texture * primary DOUBLE, texture * primary
         } 
     }
     
     // Lightmapped, encoded as dLDR
     Pass {
         Tags { "LightMode" = "VertexLM" }
         
         BindChannels {
             Bind "Vertex", vertex
             Bind "normal", normal
             Bind "texcoord1", texcoord0 // lightmap uses 2nd uv
             Bind "texcoord", texcoord1 // main uses 1st uv
         }
         
         SetTexture [unity_Lightmap] {
             matrix [unity_LightmapMatrix]
             constantColor [_Color]
             combine texture * constant
         }
         SetTexture [_MainTex] {
             combine texture * previous DOUBLE, texture * primary
         }
     }
     
     // Lightmapped, encoded as RGBM
     Pass {
         Tags { "LightMode" = "VertexLMRGBM" }
         
         BindChannels {
             Bind "Vertex", vertex
             Bind "normal", normal
             Bind "texcoord1", texcoord0 // lightmap uses 2nd uv
             Bind "texcoord1", texcoord1 // unused
             Bind "texcoord", texcoord2 // main uses 1st uv
         }
         
         SetTexture [unity_Lightmap] {
             matrix [unity_LightmapMatrix]
             combine texture * texture alpha DOUBLE
         }
         SetTexture [unity_Lightmap] {
             constantColor [_Color]
             combine previous * constant
         }
         SetTexture [_MainTex] {
             combine texture * previous QUAD, texture * primary
         }
     }
     
     // Pass to render object as a shadow caster
     Pass {
         Name "ShadowCaster"
         Tags { "LightMode" = "ShadowCaster" }
         
         Fog {Mode Off}
         ZWrite On ZTest Less Cull Off
         Offset 1, 1
 
 CGPROGRAM
 #pragma vertex vert
 #pragma fragment frag
 #pragma multi_compile_shadowcaster
 #pragma fragmentoption ARB_precision_hint_fastest
 #include "UnityCG.cginc"
 
 struct v2f { 
     V2F_SHADOW_CASTER;
 };
 
 v2f vert( appdata_base v )
 {
     v2f o;
     TRANSFER_SHADOW_CASTER(o)
     return o;
 }
 
 float4 frag( v2f i ) : COLOR
 {
     SHADOW_CASTER_FRAGMENT(i)
 }
 ENDCG
 
     }
     
     // Pass to render object as a shadow collector
     Pass
     {
         Name "ShadowCollector"
         Tags { "LightMode" = "ShadowCollector" }
         
         Fog {Mode Off}
         ZWrite On ZTest Less
 
         CGPROGRAM
                 #pragma vertex vert
                 #pragma fragment frag
                 #pragma fragmentoption ARB_precision_hint_fastest
                 #pragma multi_compile_shadowcollector
                 
                 #define SHADOW_COLLECTOR_PASS
                 #include "UnityCG.cginc"
                 
                 struct appdata {
                     float4 vertex : POSITION;
                 };
                 
                 struct v2f {
                     V2F_SHADOW_COLLECTOR;
                 };
                 
                 v2f vert (appdata v)
                 {
                     v2f o;
                     TRANSFER_SHADOW_COLLECTOR(o)
                     return o;
                 }
                 
                 half4 frag (v2f i) : COLOR
                 {
                     SHADOW_COLLECTOR_FRAGMENT(i)
                 }
         ENDCG
     }




    //###########################################################
    //###########################################################
    //###########################################################
    //###########################################################
    //###########################################################
    //###########################################################
    //###########################################################
    //########## BEGIN SECONDARY SURFACE SHADER PASS ############
    //###########################################################
    //###########################################################
    //###########################################################
    //###########################################################
    //###########################################################
    //###########################################################
    //###########################################################
     
     CGPROGRAM
         #pragma surface surf Lambert
         
         struct Input
         {
           float4 color : COLOR;
         };
         
         
         void surf (Input IN, inout SurfaceOutput o) {
           o.Albedo = 0;
           o.Alpha = 0;
         }
     ENDCG
     
     
 }
 
 // 1 texture stage GPUs
 SubShader {
     Tags { "RenderType"="Opaque" }
     LOD 100
 
     // Non-lightmapped
     Pass {
         Tags { "LightMode" = "Vertex" }
         
         Material {
             Diffuse [_Color]
             Ambient [_Color]
             Shininess [_Shininess]
             Specular [_SpecColor]
             Emission [_Emission]
         } 
         Lighting On
         SeparateSpecular On
         SetTexture [_MainTex] {
             Combine texture * primary DOUBLE, texture * primary
         } 
     }    
     // Lightmapped, encoded as dLDR
     Pass {
         Tags { "LightMode" = "VertexLM" }
 
         BindChannels {
             Bind "Vertex", vertex
             Bind "texcoord1", texcoord0 // lightmap uses 2nd uv
         }        
         SetTexture [unity_Lightmap] {
             matrix [unity_LightmapMatrix]
             constantColor [_Color]
             combine texture * constant
         }
     }
     Pass {
         Tags { "LightMode" = "VertexLM" }
         ZWrite Off
         Blend DstColor Zero
         SetTexture [_MainTex] {
             combine texture
         }
     }
 }
 }