How to achieve shadows in custom Vertex/Frag shaders?

Hi I am working on a custom vertex shader which gives the world objects a curve effect. But unfortunately when i use this shader in all the object in the scene, the objects doesn't show shadows.

I tried adding a second cast/collect shadow pass to the sub shader using the example in this manual: Vertex and Fragment shader Examples But it doesn't show shadows.

My custom vertex shaders is:

 Shader "Custom/Curve/Normal"
 {
     Properties
     {
         _MainTex ("Texture", 2D) = "white" {}
         _QOffset ("Offset", Vector) = (0,0,0,0)
         _Dist ("Distance", Float) = 100.0
     }
     SubShader
     {
         Tags { "RenderType"="Opaque" }
         LOD 100
 
         Pass
         {
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             // make fog work
             #pragma multi_compile_fog
             
             #include "UnityCG.cginc"
 
             struct appdata
             {
                 float4 vertex : POSITION;
                 float2 uv : TEXCOORD0;
             };
 
             struct v2f
             {
                 float2 uv : TEXCOORD0;
                 UNITY_FOG_COORDS(1)
                 float4 vertex : SV_POSITION;
             };
 
             sampler2D _MainTex;
             float4 _MainTex_ST, _QOffset;
             float _Dist;
              
             v2f vert (appdata v)
             {
                  v2f o;
                 float4 vPos = mul (UNITY_MATRIX_MV, v.vertex);
                 float zOff = vPos.z/_Dist;
                 vPos += _QOffset*zOff*zOff;
                 o.vertex = mul (UNITY_MATRIX_P, vPos);
 
                 o.uv = TRANSFORM_TEX(v.uv, _MainTex);
 
                 UNITY_TRANSFER_FOG(o,o.vertex);
 
                 return o;
             }
             
             fixed4 frag (v2f i) : SV_Target
             {
                 // sample the texture
                 fixed4 col = tex2D(_MainTex, i.uv);
                 // apply fog
                 UNITY_APPLY_FOG(i.fogCoord, col);
                 return col;
             }
             ENDCG
         } 
     }
     Fallback "Diffuse"
 }

I am achieving the desired curve effect in the world using the suggested technique mentioned in this link: How to make plane look curved?

How to do I achieve shadows now using another pass and also have the desired curve effect?

Unity Vesrsion Used: 5.4.2f2 Personal