Anyway when I use a dot product to check visibility with the shared verts, it doesnt always work i.e. I can physically see the vert on the screen before dot product is <=0.
1) You should probably unweld and correct the vertex normals first.
2) IIRC you need to point the camera at the vertex you are checking against. Or use a pivot as a 'dummy' camera, parented to the real camera. Then you don't have to bother restoring its rotation afterwards.
Maybe try (2) first and see if that sorts it out.