Ok, but Ubuntu stick to the vlc version known at the moment of their release , so regular install is not possible. I did not found how I could install manually the newest version (provided it would work in an environment with old version of the other libs).
I'm so happy to finaly find this page explaining my long during bug and how to tournaround it ! And it is even worse than you think: you already get it if you reset prefs to defaults, then go to prefs/all/inpus/acess/V4L2, and juste save prefs (after changing some value + restoring it immediatly jus...