How can I have the camera forward in a vertex shader?

I want to create a Rim Shader like this: http://spong.com/screen-shot/b/a/batmanarkh334306l/_-Batman-Arkham-City-Enhanced-Detective-Mode-Detailed-_.jpg

I know that is pretty simple in surface, but I need it in a vertex shader. I tried using what it says here http://answers.unity3d.com/questions/170589/shader-actual-view-directionnormal.html but it doesn't work.

Here is my code:

 Shader "Custom/Rim" 
 {
     Properties 
     {
         _Color("Color", Color) = (1,1,1,1)
     }
     SubShader 
     {
         Pass{
             Blend One OneMinusSrcAlpha
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #include "UnityCG.cginc"
     
                 float4 _Color;
                 struct input{
                     float4 vertex:POSITION;
                     float3 normal:NORMAL;
                 };
                 
                 struct output{
                     float4 vertex:SV_POSITION;
                     float3 normal;
                     float3 viewDir: TEXCOORD0;
                 };
             
                 output vert (input i){
                     output o;
                     o.vertex = mul(UNITY_MATRIX_MVP, i.vertex);
                     o.normal = i.normal;
                     //Also tried this:
                     //o.viewDir = mul((float3x3)UNITY_MATRIX_IT_MV, i.normal);
                     o.viewDir = normalize(WorldSpaceViewDir(o.vertex));
                     return o;
                 }
                 
                 
                 float4 frag (output o):COLOR{
                     half rim = 1 - dot(normalize(o.viewDir), o.normal);
                     float4 c;
                     c = _Color;
                     c.a = rim;
                     return c;
                 }
             
             
             ENDCG
         }
     }
 }