How to add lightning to a sprite billboard shader? - unity3d

Question

I'm currently working on large terrains in unity.
I need to use custom trees, that basically hold a 3d model and a sprite. With LOD component the model is disabled and sprite enabled with distance.
I founded a billboard shader on github that's match almost perfectly what's I searched for,
exepting that it isn't using lightning, so the sprite have always the same "Color" with and without light in the scene.
I'm currently trying to mix it with diffuse shader but I don't have any knowledges in shader coding.
Here is the shader in question:
Shader "Custom/BillboardSprite" {
Properties {
_MainTex ("Main Tex", 2D) = "white" {}
_Color ("Color Tint", Color) = (1, 1, 1, 1)
_VerticalBillboarding ("Vertical Restraints", Range(0, 1)) = 1
}
SubShader {
// Need to disable batching because of the vertex animation
Tags {"Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent" "DisableBatching"="True"}
Pass {
Tags { "LightMode"="ForwardBase" }
ZWrite Off
Blend SrcAlpha OneMinusSrcAlpha
Cull Off
CGPROGRAM
#pragma vertex vert
#pragma fragment frag
#include "Lighting.cginc"
sampler2D _MainTex;
float4 _MainTex_ST;
fixed4 _Color;
fixed _VerticalBillboarding;
struct a2v {
float4 vertex : POSITION;
float4 texcoord : TEXCOORD0;
};
struct v2f {
float4 pos : SV_POSITION;
float2 uv : TEXCOORD0;
};
v2f vert (a2v v) {
v2f o;
// Suppose the center in object space is fixed
float3 center = float3(0, 0, 0);
float3 viewer = mul(unity_WorldToObject,float4(_WorldSpaceCameraPos, 1));
float3 normalDir = viewer - center;
// If _VerticalBillboarding equals 1, we use the desired view dir as the normal dir
// Which means the normal dir is fixed
// Or if _VerticalBillboarding equals 0, the y of normal is 0
// Which means the up dir is fixed
normalDir.y =normalDir.y * _VerticalBillboarding;
normalDir = normalize(normalDir);
// Get the approximate up dir
// If normal dir is already towards up, then the up dir is towards front
float3 upDir = abs(normalDir.y) > 0.999 ? float3(0, 0, 1) : float3(0, 1, 0);
float3 rightDir = normalize(cross(upDir, normalDir));
upDir = normalize(cross(normalDir, rightDir));
// Use the three vectors to rotate the quad
float3 centerOffs = v.vertex.xyz - center;
float3 localPos = center + rightDir * centerOffs.x + upDir * centerOffs.y + normalDir * centerOffs.z;
o.pos = UnityObjectToClipPos(float4(localPos, 1));
o.uv = TRANSFORM_TEX(v.texcoord,_MainTex);
return o;
}
fixed4 frag (v2f i) : SV_Target {
fixed4 c = tex2D (_MainTex, i.uv);
c.rgb *= _Color.rgb;
return c;
}
ENDCG
}
}
FallBack "Transparent/VertexLit"
}