Vertex shader that casts shadows from environment on a model

Hello,

I'm struggling to make a Vertex shader which would cast environment shadow on the player's model. How to achieve it?

I can not use a diffuse Vertex shader due to loosing cartoonish look (model starts to look dirty).

Currently I am using this shader (Top Image):

 Shader "Vertex Color/Standard"
 {
     Properties
     {
         _Color ("Main Color", Color) = (1, 1, 1, 1)
     }
     SubShader
     {
         Tags {"RenderType"="Opaque"}
         Cull Off
         ZWrite On
         Fog { Mode Off }
     Pass
     {
         CGPROGRAM
         #pragma vertex vert
         #pragma fragment frag
         fixed4 _Color;
         struct appdata
         {
             float4 vertex : POSITION;
             float4 color : COLOR;
         };
         struct v2f
         {
             float4 pos : SV_POSITION;
             fixed4 color : COLOR;
         };
         v2f vert (appdata v)
         {
             v2f o;
             o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
             o.color = v.color;
             return o;
         }
         half4 frag(v2f i) : COLOR
         {
             return i.color * _Color;
         }
         
         
         ENDCG
         }
     }
     FallBack "Diffuse"
 }

Vertex Diffuse Shader (Second Image):

 Shader "Vertex Color/Diffuse" {
     Properties {
         _Color ("Main Color", Color) = (1,1,1,1)   
     }
 
     SubShader {
         Tags { "RenderType"="Opaque" }
         LOD 150
         Cull Off
         CGPROGRAM
         #pragma surface surf Lambert vertex:vert
         
         fixed4 _Color;
         
         struct Input {
             float3 vertColor;
         };
 
         void vert (inout appdata_full v, out Input o) {
             UNITY_INITIALIZE_OUTPUT(Input, o);
             o.vertColor = v.color;
         }
 
         void surf (Input IN, inout SurfaceOutput o) {
             fixed4 c = _Color;
             o.Albedo = c.rgb * IN.vertColor;
             o.Alpha = c.a;
         }
         ENDCG
     }
     Fallback "Vertex Color/Standard"
 }