Sorry, it took me some time to try different things. Thank you IOhannes for keeping this alive ^^
Hehe, Labomedia is a nice place for informations, but to handle with care. But that tutorial allowed me to have vlc plugin working some years ago, and as little changed since I don't see why it doesn't work anymore.
So far still no luck, but with my latest tries I had some interesting behaviour, that makes me think this might more be a vlc plugin issue as IOhannes suggested. Moreover Antoine kindly sent me it's binaries (those that are working on his computer, and they behave the same on my computer as the Gem I compiled. So let's say it's not related to compilation.
Looking in the plugin path direction, I realized that whatever vlc options I add with the path (like --plugin-path, or -vv), then pd doesn't crash, but doesn't display the frames. If pd is launched from terminal I get error messages from vlc core, so it seems like Gem and vlc are definitly talking to each other, and that it's a further negociation between them that causes the crash. Ubunt reports and gdb bt point at lua, it looks like lua scripts are used to parse metadata and get the plugin vlc should use, but I can't understantd why this works with vlc in cli but not in Gem.
Any idea what could be the issue ? I'm making a fresh install each and every time, vlc and vlc-dev from repo, no customisation. Even tried installing only the components suggested by Antoine on the other thread related to vlc (VlC plugin - missing openGL header)