I’m not familiar with the VectorLine class, but I would suspect it is using the GL drawing functions to draw the line. Unfortunately, all GL drawing is done AFTER the scene has been renderered, so everything drawn with GL, just goes on top.
You could use the builtin LineRenderer component, but sadly it does not have an option to draw single-pixel lines: the width of the line is defined in local or world coordinates. (it actually creates a MESH)