Intel vaapi : can't play mkv with vlc
Ok all i had to do according this https://bbs.archlinux.org/viewtopic.php?id=199749 is to launch vlc with vlc --avcodec-hw=vaapi or to change vlc settings in "codec" and choose vaapi via x11 instead of vdpau.
If you like your system clean, you can also remove vdpau drivers (packages look like vdpau-driver-all:amd64 and mesa-vdpau-drivers:amd64). This will force the use of VAAPI.