Lighting missing on back face

I am quite new to making custom shaders, really appreciate any help. I have a vertex shader that does not render lighting correctly on the back face -- the front face and back face shows different lighting. I would like the back face to have the same lighting effect as the front. I understand that a possible method making use of VFACE or passing Cull Back and Cull Front instead of Cull Off.

For VFACE, I am getting errors when I try to put "o.facing = v.facing;" into my vert function. My shader seems to be slightly different from other examples and I just can't seem to get it to work. Also, using two passes does not produce the desired effect as well. Is there something that I am doing wrongly?

Code (CSharp):

1.  
2. Shader "Custom/Vertex Transparent Specular"
3. {
4.     Properties
5.     {
6.         _Color("Main Color", Color) = (1,1,1,1)
7.         _Shininess("Shininess", Range(0, 1)) = 0.078125
8.         _Detail("Detail (RGB) Gloss (A)", 2D) = "gray" {}
9.     }
10.  
11.         SubShader
12.     {
13.         Tags { "RenderType" = "Transparent" }
14.         Blend SrcAlpha OneMinusSrcAlpha
15.         LOD 200
16.         Pass{
17.              Cull Back
18.         }
19.       CGPROGRAM
20.       #pragma surface surf BlinnPhong vertex:vert alpha:fade
21.  
22.         sampler2D _Detail;
23.         float4 _Color;
24.         float _Shininess;
25.  
26.         struct Input
27.         {
28.             float2 uv_Detail;
29.             float4 vertColors;
30.         };
31.  
32.         void vert(inout appdata_full v, out Input o)
33.         {
34.             o.vertColors = v.color;
35.             o.uv_Detail = v.texcoord;  
36.  
37.         }
38.  
39.  
40.         void surf(Input IN, inout SurfaceOutput o)
41.         {
42.             half4 c = IN.vertColors.rgba * _Color.rgba;              
43.             c.rgb *= tex2D(_Detail,IN.uv_Detail).rgb * 2;
44.             o.Albedo = c.rgb;
45.             o.Gloss = tex2D(_Detail,IN.uv_Detail).a;
46.             o.Alpha = c.a * _Color.a;
47.             o.Specular = _Shininess;
48.         }
49.  
50.         ENDCG
51.     }
52.  
53.         Fallback "Specular"
54. }
55.  
56.