How do you get the world position of a vertex in a shader?

I am trying to get the world position of a vertex into my fragment shader but the _Object2World translation doesn't appear to be working. Is there something I need to do in scripting such that the _Object2World matrix is updated?

Below is my simplified shader. We are using the 3.4.2f3 version of unity. My scene for this test contains:

plane pos: 0,0,100 rotation: 270,0,0 scale: 5,5,5

cube pos: 0,0,20 rotation: 0,0,0 scale: 1,1,1

camera pos: 0,0,0 rotation: 0,0,0 scale: 1,1,1

The result of this shader when applied to the scene is always red. I would expect to see green because the wpos.z should be in world coordinates and the vpos.z would still be in object coordinates. Note that I did see a similar question("How to get the global position of a vertex in a Cg vertex shader?") and have tried to make my vertex shader do exactly the same thing as the response to that question.

Help!

Shader "DepthEffect" { Properties { _MainTex ("Base (RGB)", 2D) = "white" {} } SubShader { Pass { ZTest Always Cull Off ZWrite Off Fog { Mode off }

         CGPROGRAM
         #pragma vertex vert
         #pragma fragment frag
         #pragma fragmentoption ARB_precision_hint_fastest 
         #include "UnityCG.cginc"

         sampler2D _MainTex;
     
         struct v2f 
         {
             float4 pos  : POSITION;
             float2 uv   : TEXCOORD0;
             float3 wpos : TEXCOORD1;
             float3 vpos : TEXCOORD2;
         };
         
         v2f vert( appdata_img v )
         {
             v2f o;
             o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
             float3 worldPos = mul (_Object2World, v.vertex).xyz;
             o.wpos = worldPos;
             o.vpos = v.vertex.xyz;
             o.uv =  v.texcoord.xy;
             return o;
         }
         
         float4 frag (v2f i) : COLOR
         {
             if (i.vpos.z == i.wpos.z)
                 return float4(1,0,0,1);
                 else
                 return float4(0,1,0,1);
         }
         ENDCG
     }
 } 
 FallBack "Diffuse"

}