moving vertices along view axis between camera and vertex position I do want to model something that i a have photographed in 2 different perspectives.
So after modeling it looking through one camera, i do want to slide the vertices back and forth along the vector describing the direction between camera origin and vertex position (and not the view axis, because that would always be the same direction for every pixel of the image)
It's like creating a vertex while viewing from a fixed position/rotation, than making an edge to the viewing origin and vertex sliding the first vertex without clamp.
Create a plane and delete all but one vertex then move the origin of the plane to this vertex. Copy the camera coordinates to this vertex object so they are in the same position. On your target object extrude a vertex and snap it to the camera vertex. Now you have an edge between the camera and target vertex that you can subdivide and vertex slide using G-G.