Shader distance between WorldSpaceCameraPos and Vertex

I have resolved in another way. I have created a simple alpha emissive shader which depends on vertices colors:

 Shader "Custom/MyShader"
 {
     Properties
     {
         _Color("Main Color", Color) = (1,1,1,1)
         _mul("Multiplier", Float) = 1
         _addSubtract("Add / Subtract", Float) = 0
         _alpha("Alpha", Range(0,1)) = .5
         _MainTex("Base (RGB)", 2D) = "white"{}
         _AlphaMap("Alpha Texture (RGB)", 2D) = "white"{}
     }
     
     SubShader
     {
         Tags { "Queue"="Transparent" }
         
         CGPROGRAM
         #pragma surface surf Lambert alpha
 
         float4 _Color;
         float _mul;
         float _addSubtract;
         float _alpha;
         sampler2D _MainTex;
         sampler2D _AlphaMap;
 
         struct Input
         {
             float2 uv_MainTex;
             float2 uv_AlphaMap;
             float4 color : Color;
         };
         
         void surf (Input IN, inout SurfaceOutput o)
         {
             o.Emission = (tex2D(_MainTex, IN.uv_MainTex) * _mul + _addSubtract) * _Color.rgb * IN.color.rgb;
             o.Alpha = saturate(tex2D(_AlphaMap, IN.uv_AlphaMap)) * _alpha * IN.color.a;
         }
         ENDCG
     } 
     FallBack "Diffuse"
 }

Then i have created a C# Script that controls the vertices colors in relation to the player position:

 using UnityEngine;
 using System.Collections;
 
 public class VertexCtrl : MonoBehaviour
 {
     
     public float dist = 20;
     Mesh mesh;
     Vector3[] vertices;
     Color[] colors;
     
     void Start()
     {
         mesh = GetComponent<MeshFilter>().mesh;
         vertices = mesh.vertices;
         colors = new Color[vertices.Length];
     }
 
     void Update()
     {
         foreach(GameObject obj in FindSceneObjectsOfType(typeof(GameObject)))
         {
             if(obj.tag == "Player")
             {
                 for(int i = 0; i < vertices.Length; i++)
                 {
                     if(Vector3.Distance(transform.TransformPoint(vertices[i]), obj.transform.position) > dist)
                         colors[i] = new Color(0, 0, 0, 0);
                     else
                         colors[i] = new Color(0, 0, 0, 1 - Vector3.Distance(transform.TransformPoint(vertices[i]), obj.transform.position) / dist);
                 }
                 mesh.colors = colors;
             }
         }
     }
 }

It works very well for me. I hope it helps someone...