vertex shader with alpha channel

Hello! I have been working on a space exploration game for the past month or so. The planets in this game are procedurally generated and the color of each vertex is dependent on the height. Unfortunately, I have run into a problem with creating a seamless Level Of Detail (LOD) for my planets. Each of my LOD's is a sphere that has less vertices than the LOD beneath it. When I get a certain distance from my planet, one of the LOD's gets disabled and another one enabled. This creates an ugly popping as the Level of Details are switched. What I want to do is have an alpha channel in my vertex shader so that I can fade out the LOD's instead of instantly enabling/disabling them. I do not need each vertex to have an individual transparency, just the entire mesh as a whole. Here is the shader I am using:

 Shader "Simple Diffuse" {
 Properties {
     _MainTex ("Texture", 2D) = "white" {}
 }
  
 Category {
     Tags { "Queue"="Geometry" }
     Lighting Off
     BindChannels {
         Bind "Color", color
         Bind "Vertex", vertex
         Bind "TexCoord", texcoord
     }
  
     SubShader {
         Pass {
             SetTexture [_MainTex] {
                 Combine texture * primary DOUBLE
             }
         }
     }
 }
 }

Thanks in advance, - Mark