Z-priming alpha-per-vertex

I'm using the MeshParticleRenderer from asset-store, which renders all particles together as one mesh. It is able to set color-animation on vertex-base, and by using the custom shader it is able to do correct color animations. However, transparency problems and wrong render orders appear because of the Z-write alpha problem. A solution would be Z-priming, as I added in this shader. However, Z-priming pass only puts "full black" in the z-buffer. If a particle would fade in and out, the particle immediately turns black (non-transparent) and then starts fading in the color. When fading out, it first fades out to black (non-transparent), then immediately disappears.

Is there any way I can make the Z-prime pass use the alpha-per-vertex values?

 Shader "Internal/ParticleVertex" {
     Properties {
       _MainTex ("Texture", 2D) = "white" {}
       _BumpMap ("Texture", 2D) = "bump" {}
    _Color ("Main Color", Color) = (1,1,1,1)
    _SpecColor ("Specular Color", Color) = (1.0, 1.0, 1.0, 1.0)
     }
     SubShader {
  // Zprime Frontfaces
  Pass {
  Tags {"Queue"="Transparent" "IgnoreProjector"="True" "LightMode" = "Always"}
  zwrite on Cull off
  colormask 0
  }
  
  // Front Faces
  Tags {"Queue"="Transparent" "RenderType"="Transparent" }
       CGPROGRAM
       #pragma surface surf BlinnPhong vertex:vert alpha
       
       struct Input {
         float2 uv_MainTex;
         float4 vertexColour;
       };
       
       
       void vert (inout appdata_full v, out Input o)
       {
           o.vertexColour = v.color;
       }
             
       sampler2D _MainTex;
 
       void surf (Input IN, inout SurfaceOutput o) {
         o.Albedo = tex2D (_MainTex, IN.uv_MainTex).rgb;
         o.Albedo *= IN.vertexColour.rgb;
         o.Alpha = tex2D (_MainTex, IN.uv_MainTex).a;
         o.Alpha *= IN.vertexColour.a;
 o.Specular = 0.2f;
  o.Gloss = 1.5f;
       }
       ENDCG
     }
     Fallback "Transparent/Diffuse"
  }