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"
}

The answer appears to be that previously, Unity did not care if the Properties of the Fallback shader were a subset of the main shader. Now it does. So, in the above case, adding:

_Color ("Main Color", Color) = (1,1,1,1)

Fixes the problem (even though the main shader does not use _Color). This of course makes perfect sense, as it ensures properties are passed to the fallback shader.