Split Standard PBS and Simple lighting vertex shaders as shininess was being used in PBS.

ScriptableRenderPipeline/LightweightPipeline/LWRP/ShaderLibrary/LightweightPassLit.hlsl

 //                  Vertex and Fragment functions                            //
 ///////////////////////////////////////////////////////////////////////////////
 
-// Vertex: Used for Standard and StandardSimpleLighting shaders
+// Used in Standard (Physically Based) shader
 LightweightVertexOutput LitPassVertex(LightweightVertexInput v)
 {
     LightweightVertexOutput o = (LightweightVertexOutput)0;
     o.uv = TransformMainTextureCoord(v.texcoord);
 
     o.posWSShininess.xyz = TransformObjectToWorld(v.vertex.xyz);
-    o.posWSShininess.w = _Shininess * 128.0;
     o.clipPos = TransformWorldToHClip(o.posWSShininess.xyz);
 
     half3 viewDir = VertexViewDirWS(GetCameraPositionWS() - o.posWSShininess.xyz);
     return o;
 }
 
-// Used for Standard shader
+// Used in Standard (Physically Based) shader
 half4 LitPassFragment(LightweightVertexOutput IN) : SV_Target
 {
     UNITY_SETUP_INSTANCE_ID(IN);
 
     ApplyFog(color.rgb, inputData.fogCoord);
     return color;
+}
+
+// Used in Standard (Simple Lighting) shader
+// TODO: we only need to specialise this because of _Shininess prop
+// Once we refactor SimpleLighting shader we will be able to share vertex
+// between PBS and Simple
+LightweightVertexOutput LitPassVertexSimple(LightweightVertexInput v)
+{
+    LightweightVertexOutput o = (LightweightVertexOutput)0;
+
+    UNITY_SETUP_INSTANCE_ID(v);
+    UNITY_TRANSFER_INSTANCE_ID(v, o);
+    UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(o);
+
+    o.uv = TransformMainTextureCoord(v.texcoord);
+
+    o.posWSShininess.xyz = TransformObjectToWorld(v.vertex.xyz);
+    o.posWSShininess.w = _Shininess * 128.0;
+    o.clipPos = TransformWorldToHClip(o.posWSShininess.xyz);
+
+    half3 viewDir = VertexViewDirWS(GetCameraPositionWS() - o.posWSShininess.xyz);
+
+#ifdef _NORMALMAP
+    o.normal.w = viewDir.x;
+    o.tangent.w = viewDir.y;
+    o.binormal.w = viewDir.z;
+#else
+    o.viewDir = viewDir;
+#endif
+
+    // initializes o.normal and if _NORMALMAP also o.tangent and o.binormal
+    OUTPUT_NORMAL(v, o);
+
+    // We either sample GI from lightmap or SH.
+    // Lightmap UV and vertex SH coefficients use the same interpolator ("float2 lightmapUV" for lightmap or "half3 vertexSH" for SH)
+    // see DECLARE_LIGHTMAP_OR_SH macro.
+    // The following funcions initialize the correct variable with correct data
+    OUTPUT_LIGHTMAP_UV(v.lightmapUV, unity_LightmapST, o.lightmapUV);
+    OUTPUT_SH(o.normal.xyz, o.vertexSH);
+
+    half3 vertexLight = VertexLighting(o.posWSShininess.xyz, o.normal.xyz);
+    half fogFactor = ComputeFogFactor(o.clipPos.z);
+    o.fogFactorAndVertexLight = half4(fogFactor, vertexLight);
+
+#ifdef _SHADOWS_ENABLED
+#if SHADOWS_SCREEN
+    o.shadowCoord = ComputeShadowCoord(o.clipPos);
+#else
+    o.shadowCoord = TransformWorldToShadowCoord(o.posWSShininess.xyz);
+#endif
+#endif
+
+    return o;
 }
 
 // Used for StandardSimpleLighting shader

ScriptableRenderPipeline/LightweightPipeline/LWRP/Shaders/LightweightStandardSimpleLighting.shader

             // GPU Instancing
             #pragma multi_compile_instancing
 
-            #pragma vertex LitPassVertex
+            #pragma vertex LitPassVertexSimple
             #pragma fragment LitPassFragmentSimple
             #define BUMP_SCALE_NOT_SUPPORTED 1
             #include "LWRP/ShaderLibrary/LightweightPassLit.hlsl"