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"
}