Adding cutout to a legacy shader

Hi, I have no idea how simple is to achieve what I'm looking, I did some basic Legacy shaders (the ones with SetTexture), I don't know much about how turning them into fragment shaders so just wanted to know if I can do like the transparent cutout shaders with one of these? just adding the slider and "erasing" the ones that have transparency as I move the slider, my shader is just a dual texture one, one in each UV set, so I want to edit the second one, I mean apply the alpha cutout just to the second texture but have no clue how, this is the shader:

Code (CSharp):

1. Shader "Custom/Decal (2 UV sets)" {
2. Properties {
3.     _MainTex ("Base (RGB)", 2D) = "white" {}
4.     _DecalTex ("Decal (RGBA)", 2D) = "white" {} //<---this is the second texture, which supports tranparency
5. }
6.  
7. SubShader {
8.     Tags { "RenderType"="Opaque" }
9.     Material {
10.         Diffuse [_Color]
11.         Ambient [_Color]
12.    
13.     }
14.     Pass {
15.         BindChannels {
16.             Bind "Vertex", vertex
17.             Bind "normal", normal
18.             Bind "texcoord", texcoord0 // main uses 1st uv
19.             Bind "texcoord1", texcoord1 // decal uses 2nd uv
20.         }
21.         Lighting On
22.         ColorMaterial AmbientAndDiffuse
23.      
24.         SetTexture [_MainTex] {combine texture}
25.         SetTexture [_DecalTex] {combine texture lerp (texture) previous}
26.         SetTexture [_MainTex] {combine previous * primary DOUBLE} //vertex colouring
27.     }
28. }
29.  
30. Fallback " VertexLit", 1
31.  
32. }

Thanks a lot!