Transparent Vertex Shader blend with color?

Like this?

 Shader "AShader" {
 Properties {
     _Color ("Main Color", Color) = (1,1,1,1)
     _MainTex ("Base (RGB) Alpha (A)", 2D) = "white" {}
     _Cutoff ("Alpha cutoff", Range(0,1)) = 0.5
 }
  
 SubShader {
     Tags {"Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent" "LightMode"="ForwardBase"}
     LOD 100
     Cull Off
     ZWrite Off
     Blend SrcAlpha OneMinusSrcAlpha 
  
  
     CGINCLUDE
     #include "UnityCG.cginc"
     sampler2D _MainTex;
     half4 _MainTex_ST;
     half4 _Color;
  
     float4 _GrassWind; //is not defined in terrainengine.cginc
  
     struct v2f {
        float4 pos : SV_POSITION;
        float2 uv : TEXCOORD0;
     };
  
 inline float4 AnimateGrass(float4 pos, float3 normal, float animParams)
 {   
     pos.xyz += animParams * _GrassWind.xyz * _GrassWind.w; // controlled by vertex color blue
     return pos;
 }
  
     v2f vert (appdata_full v)
     {
        v2f o;
        float4    windParams = float(v.color.b);     
  
        float4 mdlPos = AnimateGrass(v.vertex, v.normal, windParams);
        o.pos = mul(UNITY_MATRIX_MVP,mdlPos);
        o.uv = TRANSFORM_TEX(v.texcoord, _MainTex);
        return o;
     }
     ENDCG
  
     Pass {
        CGPROGRAM
        #pragma debug
        #pragma vertex vert
        #pragma fragment frag
        #pragma fragmentoption ARB_precision_hint_fastest     
        fixed4 frag (v2f i) : COLOR
        {
          fixed4 tex = tex2D (_MainTex, i.uv);
  
          fixed4 c;
          c.rgb = tex.rgb;
          c.a = tex.a;
  
          return c * _Color;
        }
        ENDCG 
     }  
 }
 }

You have to declare the color like this:

     sampler2D _MainTex;
     half4 _MainTex_ST;
     half4 _Color; // see the color added to these lines.

Then you can just multiply the output color with the color you want it blended with:

 return c * _Color;