What's the best way to read a texture in camera space in a vertex shader?

Hi, I'm currently trying to write a shader w$$anonymous$$ch will distort the scene in front of it using a vertex function and an image. (With the default particle texture for example t$$anonymous$$s shader might cause vertices in the middle of the camera to buldge). I'm struggling to get the texture to be a constant size on the screen even with (as far as I can tell) correct transformations for screenposition. Could someone give me a second pair of eyes on t$$anonymous$$s?

         //screenspace effect
         float4 clipSpace = UnityObjectToClipPos(v.vertex);
         float4 screenSpace = ComputeScreenPos(clipSpace);
         float4 deformTex = tex2Dlod(_ScreenspaceDeformTex, float4((((screenSpace.xy) / screenSpace.z) / _ScreenParams.yx), 0, 0)).r;

I've written fragment shader functions using VPOS w$$anonymous$$ch are very accurate but haven't found an analog in the vertex shader world to help solve t$$anonymous$$s problem.