Zulfiqar you wrote: "You are in no particular space in fragment shaders!"
You're right, but what I was trying to mean is that all the information calculated by GL for the fragment shader (like half-vector) is done in eye-view space.
Trahern, could you develop why specular lighting could be a problem on vertex-lighting ? At this point, I don't see why.
To both of you, I now understand more all the process, so I'll surely (soon or later) also implement a vertex-lighting version to try out efficiency and difference between the two rendering results.
Thank you for having taking time for my problems