Add fog to vertex shader

I've been trying to modify a shader to be effected by Unity's exponential fog. I'm not really verse in shader code, and I've been searching through previous answers. I think it has something with adding the " #pragma multi_compile_fog" and then adding the values within the shader itself, but I've been getting no where or a bunch of error, so I figure I could ask.

Here is the code:

 Shader "Custom/Outline Fill" {
   Properties {
     [Enum(UnityEngine.Rendering.CompareFunction)] _ZTest("ZTest", Float) = 0
 
     _OutlineColor("Outline Color", Color) = (1, 1, 1, 1)
     _OutlineWidth("Outline Width", Range(0, 10)) = 2
   }
 
   SubShader {
     Tags {
       "Queue" = "Transparent+110"
       "RenderType" = "Transparent"
       "DisableBatching" = "True"
     }
 
     Pass {
       Name "Fill"
       Cull Off
       ZTest [_ZTest]
       ZWrite Off
       Blend SrcAlpha OneMinusSrcAlpha
       ColorMask RGB
 
       Stencil {
         Ref 1
         Comp NotEqual
       }
 
       CGPROGRAM
       #include "UnityCG.cginc"
 
       #pragma vertex vert
       #pragma fragment frag
       #pragma multi_compile_fog
 
       struct appdata {
         float4 vertex : POSITION;
         float3 normal : NORMAL;
         float3 smoothNormal : TEXCOORD3;
         UNITY_VERTEX_INPUT_INSTANCE_ID
       };
 
       struct v2f {
         float4 position : SV_POSITION;
         fixed4 color : COLOR;
         UNITY_VERTEX_OUTPUT_STEREO
       };
 
       uniform fixed4 _OutlineColor;
       uniform float _OutlineWidth;
 
       v2f vert(appdata input) {
         v2f output;
 
         UNITY_SETUP_INSTANCE_ID(input);
         UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(output);
 
         float3 normal = any(input.smoothNormal) ? input.smoothNormal : input.normal;
         float3 viewPosition = UnityObjectToViewPos(input.vertex);
         float3 viewNormal = normalize(mul((float3x3)UNITY_MATRIX_IT_MV, normal));
 
         output.position = UnityViewToClipPos(viewPosition + viewNormal * -viewPosition.z * _OutlineWidth / 1000.0);
         output.color = _OutlineColor;
 
         return output;
       }
 
       fixed4 frag(v2f input) : SV_Target {
         return input.color;
       }
       ENDCG
     }
   }
 }