UNITY_VERTEX_INPUT_INSTANCE_ID use in custom shader

Question

I am using an old shader used for a character model from the Unity asset store that has depreciated. The asset has been made free and some people have put out some updated versions but no one has put out a shader that works in VR without enabling the multipass.

I have looked at Unity's guide for amending shaders for VR: text

I have tied to follow this guide with my little to none experience with Shaders but I cannot get it to work.

Can anyone help me to update the shader code with the Unity example code so it works with VR in Unity?

Any help would be greatly appreciated.

The shader code I have is below:

Shader "MCS/Volund Variants/Standard Character (Specular, Surface)"
{
    Properties
    {
        _Color("Color", Color) = (1,1,1,1)
        _MainTex("Albedo", 2D) = "white" {}
        _AlphaTex("Alpha", 2D) = "white" {}
        
        _Cutoff("Alpha Cutoff", Range(0.0, 1.0)) = 0.5

        _Glossiness("Smoothness", Range(0.0, 1.0)) = 0.5
        _SpecColor("Specular", Color) = (0.2,0.2,0.2)
        _SpecGlossMap("Specular", 2D) = "white" {}

        _BumpScale("Scale", Float) = 1.0
        _BumpMap("Normal Map", 2D) = "bump" {}

        _Parallax ("Height Scale", Range (0.005, 0.08)) = 0.02
        _ParallaxMap ("Height Map", 2D) = "black" {}

        _OcclusionStrength("Strength", Range(0.0, 1.0)) = 1.0
        _OcclusionMap("Occlusion", 2D) = "white" {}

        _EmissionColor("Color", Color) = (0,0,0)
        _EmissionMap("Emission", 2D) = "white" {}
        
        _DetailMask("Detail Mask", 2D) = "white" {}

        _DetailAlbedoMap("Detail Albedo x2", 2D) = "grey" {}
        _DetailNormalMapScale("Scale", Float) = 1.0
        _DetailNormalMap("Normal Map", 2D) = "bump" {}

        [Enum(UV0,0,UV1,1)] _UVSec ("UV Set for secondary textures", Float) = 0

        // UI-only data
        [HideInInspector] _EmissionScaleUI("Scale", Float) = 0.0
        [HideInInspector] _EmissionColorUI("Color", Color) = (1,1,1)

        // Blending state
        [HideInInspector] _Mode ("__mode", Float) = 0.0
        [HideInInspector] _SrcBlend ("__src", Float) = 1.0
        [HideInInspector] _DstBlend ("__dst", Float) = 0.0
        [HideInInspector] _ZWrite ("__zw", Float) = 1.0

        // Volund properties
        [HideInInspector] _CullMode ("__cullmode", Float) = 2.0
        [HideInInspector] _SmoothnessInAlbedo ("__smoothnessinalbedo", Float) = 0.0
        _SmoothnessTweak1("Smoothness Scale", Range(0.0, 4.0)) = 1.0
        _SmoothnessTweak2("Smoothness Bias", Range(-1.0, 1.0)) = 0.0
        _SpecularMapColorTweak("Specular Color Tweak", Color) = (1,1,1,1)
        _PlaneReflectionBumpScale("Plane Reflection Bump Scale", Range(0.0, 1.0)) = 0.4
        _PlaneReflectionBumpClamp("Plane Reflection Bump Clamp", Range(0.0, 0.15)) = 0.05
    }

    CGINCLUDE
        #define UNITY_SETUP_BRDF_INPUT SpecularSetup
        #define USE_SMOOTHNESS_TWEAK
    ENDCG

    SubShader
    {
        Tags {
            "RenderType"="Opaque"
            "PerformanceChecks"="False"
        }
        LOD 300
    

        // ------------------------------------------------------------------
        //  Base forward pass (directional light, emission, lightmaps, ...)
        Pass
        {
            Name "FORWARD" 
            Tags { "LightMode" = "ForwardBase" }

            Blend [_SrcBlend] [_DstBlend]
            ZWrite [_ZWrite]
            Cull [_CullMode]

            CGPROGRAM
            #pragma target 3.0
            //#pragma only_renderers d3d11 d3d9 opengl glcore
            
            // -------------------------------------
                    
            #pragma shader_feature _NORMALMAP
            #pragma shader_feature _ _ALPHATEST_ON _ALPHABLEND_ON _ALPHAPREMULTIPLY_ON
            #pragma shader_feature _EMISSION
            #pragma shader_feature _SPECGLOSSMAP
            #pragma shader_feature ___ _DETAIL_MULX2
            #pragma shader_feature _AlphaTex;
            
            // Volund variants
            #pragma shader_feature SMOOTHNESS_IN_ALBEDO

            #pragma multi_compile_fwdbase nolightmap
            #pragma multi_compile_fog
                
            #pragma vertex vertForwardBase
            #pragma fragment fragForwardBase

            #include "Volund_UnityStandardCore.cginc"

            ENDCG
        }
        // ------------------------------------------------------------------
        //  Additive forward pass (one light per pass)
        Pass
        {
            Name "FORWARD_DELTA"
            Tags { "LightMode" = "ForwardAdd" }
            Blend [_SrcBlend] One
            Fog { Color (0,0,0,0) } // in additive pass fog should be black
            ZWrite Off
            ZTest LEqual
            Cull [_CullMode]

            CGPROGRAM
            #pragma target 3.0
            //#pragma only_renderers d3d11 d3d9 opengl glcore

            // -------------------------------------

            
            #pragma shader_feature _NORMALMAP
            #pragma shader_feature _ _ALPHATEST_ON _ALPHABLEND_ON _ALPHAPREMULTIPLY_ON
            #pragma shader_feature _SPECGLOSSMAP
            #pragma shader_feature ___ _DETAIL_MULX2

            
            // Volund variants
            #pragma shader_feature SMOOTHNESS_IN_ALBEDO

            #pragma multi_compile_fwdadd_fullshadows
            #pragma multi_compile_fog
            
            #pragma vertex vertForwardAdd
            #pragma fragment fragForwardAdd

            #include "Volund_UnityStandardCore.cginc"

            ENDCG
        }
        // ------------------------------------------------------------------
        //  Shadow rendering pass
        Pass {
            Name "ShadowCaster"
            Tags { "LightMode" = "ShadowCaster" }
            
            ZWrite On ZTest LEqual
            Cull [_CullMode]

            CGPROGRAM
            #pragma target 3.0
            //#pragma only_renderers d3d11 d3d9 opengl glcore
            
            // -------------------------------------


            #pragma shader_feature _ _ALPHATEST_ON _ALPHABLEND_ON _ALPHAPREMULTIPLY_ON
            #pragma multi_compile_shadowcaster

            #pragma vertex vertShadowCaster
            #pragma fragment fragShadowCaster

            #include "UnityStandardShadow.cginc"

            ENDCG
        }
    }
    
    FallBack "Morph3D/Volund Variants/Standard Character (Legacy)"
    CustomEditor "VolundMultiStandardShaderGUI"
}