Fog not working in my own Vertext Shader

Fog not working in my own Vertext Shader,i use default surface shader is working.

I see the documentation:

 Fog handling was changed
 
 Unity 5.0 makes built-in Fog work on WP8 and consoles, but in order to achieve that we’ve changed how Fog is done a bit. For surface shaders and fixed function shaders, nothing extra needs to be done - fog will be added automatically (you can add “nofog” to surface shader #pragma line to explicitly make it not support fog).
 
 For manually written vertex/fragment shaders, fog does not happen automagically now. You need to add
 #pragma multi_compile_fog and fog handling macros to your shader code. Check out built-in shader source, for example Unlit-Normal how to do it.

i add the #pragma multi_compile_fog , but still not working...

there is my code:

  Properties {
         _MainTex ("Base (RGB)", 2D) = "white" {}
         _QOffset ("Offset", Vector) = (0,0,0,0)
         _Dist ("Distance", Float) = 100.0
     }
   
     SubShader {
         Tags { "RenderType"="Opaque"  }
         Fog  {Mode Linear }
         Pass
         {
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #include "UnityCG.cginc"
             #pragma multi_compile_fog
              
   
             sampler2D _MainTex;
             float4 _QOffset;
             float _Dist;
   
             struct v2f {
                 float4 pos : SV_POSITION;
                 float4 uv : TEXCOORD0;
             };
   
             v2f vert (appdata_base v)
             {
                 v2f o;
                 float4 vPos = mul (UNITY_MATRIX_MV, v.vertex);
                 float zOff = vPos.z/_Dist;
                 vPos += _QOffset*zOff*zOff;
                 o.pos = mul (UNITY_MATRIX_P, vPos);
                 o.uv = mul( UNITY_MATRIX_TEXTURE0, v.texcoord );
                 return o;
             }
   
             half4 frag (v2f i) : COLOR
             {
                 half4 col = tex2D(_MainTex, i.uv.xy);
                 return col;
             }
   
             ENDCG
         }
     }