pass value from vertex to fragment without interpolation ?!

Hi, I need to pass the actual vertex coordinate from my vertex to my fragment shader without interpolation. I tried using a uniform variable but for some reason it seems not to hold a valid value at all.

An example:

 Shader "Vertex Colored" {
 Properties {
     _Color ("Main Color", Color) = (1,1,1,1)
 }
 
 SubShader {
     Pass {
 
         CGPROGRAM
 
         #pragma vertex vert
         #pragma fragment frag
         #include "UnityCG.cginc"
         float3 _p1,_p2;
         struct v2f {
             float4 pos : SV_POSITION; 
             float3 color : COLOR0;
             float3 bc : TEXCOORD2;
             float3 p : TEXCOORD3;
         };
         float PtoLine(float3 a, float3 b, float3 p)
         {
             return length(cross(p-a, p-b))/length(b-a);
         }
         v2f vert (appdata_full v) 
         {
             
             v2f o;
             o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
             _p1 = v.vertex.xyz;
             _p2 = v.tangent.xyz;
             o.p = v.vertex;
             o.color = v.color;
             o.bc = v.tangent.xyz;
             return o;
         }
         
         half4 frag (v2f i) : COLOR
         {
             //return half4(i.bc, 1);
             return half4(_p1,1);
             if(1>0) 
                 return half4(0,0,0,1);
             else 
                 return half4 (i.color, 1);
         }
         
         ENDCG 
 
     }
 }
 
 
 
 Fallback " VertexLit", 1
 }

This will ALWAYS render black, no matter what the vertex coordinate is. What is going on here ?