Shader Fallback changed in 3.4?

In 3.3, this shader would fallback to Diffuse when the camera was VertexLit, but now with 3.4 it doesn't (and gives a black result). What exactly determines when the Fallback is taken?

(sorry about the formatting, it is Strumpy Shader Editor generated)

 Shader "DiffuseRimEmission"
 {
     Properties 
     {
 _MainTex("Main Texure", 2D) = "white" {}
 _RimColor("Rim Color", Color) = (0,0,0,1)
 _RimPower("Rim Power", Range(0.1,3) ) = 1.8
 _SpecPower("Spec Power", Range(0.1,1) ) = 0.9
 
     }
     
     SubShader 
     {
         Tags
         {
 "Queue"="Geometry"
 "IgnoreProjector"="False"
 "RenderType"="Opaque"
 
         }
 
         
 Cull Back
 ZWrite On
 ZTest LEqual
 ColorMask RGBA
 Fog{
 }
 
 
         CGPROGRAM
 #pragma surface surf BlinnPhongEditor  vertex:vert
 #pragma target 2.0
 
 
 sampler2D _MainTex;
 float4 _RimColor;
 float _RimPower;
 float _SpecPower;
 
             struct EditorSurfaceOutput {
                 half3 Albedo;
                 half3 Normal;
                 half3 Emission;
                 half3 Gloss;
                 half Specular;
                 half Alpha;
             };
             
             inline half4 LightingBlinnPhongEditor_PrePass (EditorSurfaceOutput s, half4 light)
             {
 float4 Multiply1=float4(s.Albedo, 1.0) * light;
 float4 Splat0=light.w;
 float4 Multiply0=float4(s.Gloss, 1.0) * Splat0;
 float4 Multiply2=light * Multiply0;
 float4 Add2=Multiply1 + Multiply2;
 float4 Mask1=float4(Add2.x,Add2.y,Add2.z,0.0);
 float4 Luminance1= float4( Luminance( Multiply0.xyz ) );
 float4 Add0=Luminance1 + float4(s.Albedo, 1.0);
 float4 Mask0=float4(0.0,0.0,0.0,Add0.w);
 float4 Add1=Mask1 + Mask0;
 return Add1;
 
             }
 
             inline half4 LightingBlinnPhongEditor (EditorSurfaceOutput s, half3 lightDir, half3 viewDir, half atten)
             {
                 half3 h = normalize (lightDir + viewDir);
                 
                 half diff = max (0, dot (s.Normal, lightDir));
                 
                 float nh = max (0, dot (s.Normal, h));
                 float spec = pow (nh, s.Specular*128.0);
                 
                 half4 res;
                 res.rgb = _LightColor0.rgb * diff * atten * 2.0;
                 res.w = spec * Luminance (_LightColor0.rgb);
 
                 return LightingBlinnPhongEditor_PrePass( s, res );
             }
             
             struct Input {
                 float2 uv_MainTex;
 float3 viewDir;
 
             };
 
 
             void vert (inout appdata_full v, out Input o) {
 float4 VertexOutputMaster0_0_NoInput = float4(0,0,0,0);
 float4 VertexOutputMaster0_1_NoInput = float4(0,0,0,0);
 float4 VertexOutputMaster0_2_NoInput = float4(0,0,0,0);
 float4 VertexOutputMaster0_3_NoInput = float4(0,0,0,0);
 
 
             }
             
 
             void surf (Input IN, inout EditorSurfaceOutput o) {
                 o.Normal = float3(0.0,0.0,1.0);
                 o.Alpha = 1.0;
                 o.Albedo = 0.0;
                 o.Emission = 0.0;
                 o.Gloss=0.0;
                 o.Specular=0.0;
                 
 float4 Sampled2D0=tex2D(_MainTex,IN.uv_MainTex.xy);
 float4 Fresnel0_1_NoInput = float4(0,0,1,1);
 float4 Fresnel0=float4( 1.0 - dot( normalize( float4(IN.viewDir, 1.0).xyz), normalize( Fresnel0_1_NoInput.xyz ) ) );
 float4 Pow0=pow(Fresnel0,_RimPower.xxxx);
 float4 Multiply0=_RimColor * Pow0;
 float4 Master0_1_NoInput = float4(0,0,1,1);
 float4 Master0_5_NoInput = float4(1,1,1,1);
 float4 Master0_6_NoInput = float4(1,1,1,1);
 o.Albedo = Sampled2D0;
 o.Emission = Multiply0;
 o.Specular = _SpecPower.xxxx;
 o.Gloss = float4( Sampled2D0.a);
 
                 o.Normal = normalize(o.Normal);
             }
         ENDCG
     }
     Fallback "Diffuse"
 }