Generate video h264 file (however I get the same behavior for every file I tested).
$ ffmpeg -f lavfi -i color=white;fade=t=out:d=10 -t 10 -c:v libx264 test.mkv

Within the "Ubuntu" session

Selecting the option "Decoder VA-API throught DRM"
$ vlc test.mkv 
VLC media player 2.2.6 Umbrella (revision 2.2.6-0-g1aae78981c)
[000055d996caf9d8] core libvlc: Esecuzione di vlc con l'interfaccia predefinita. Usa 'cvlc' per utilizzare vlc senza interfaccia.
libva info: VA-API version 0.40.0
libva info: va_getDriverName() returns 1
libva error: va_getDriverName() failed with operation failed,driver_name=i965
[00007f9ecc000c18] vaapi_drm generic error: Failed to initialize the VAAPI device
Failed to open VDPAU backend libvdpau_nvidia.so: impossibile aprire il file oggetto condiviso: File o directory non esistente

Within the "Ubuntu on Xorg" session

Selecting the option "Decoder VA-API throught X11"
$ vlc test.mkv 
VLC media player 2.2.6 Umbrella (revision 2.2.6-0-g1aae78981c)
[0000563a5bfb79d8] core libvlc: Esecuzione di vlc con l'interfaccia predefinita. Usa 'cvlc' per utilizzare vlc senza interfaccia.
libva info: VA-API version 0.40.0
libva info: va_getDriverName() returns -1
libva error: va_getDriverName() failed with unknown libva error,driver_name=(null)
[00007f8ff4000c18] vaapi_x11 generic error: Failed to initialize the VAAPI device
Failed to open VDPAU backend libvdpau_nvidia.so: impossibile aprire il file oggetto condiviso: File o directory non esistente
QObject::~QObject: Timers cannot be stopped from another thread

Selecting the option "Decoder VA-API throught DRM"
$ vlc test.mkv 
VLC media player 2.2.6 Umbrella (revision 2.2.6-0-g1aae78981c)
[000055adbab999d8] core libvlc: Esecuzione di vlc con l'interfaccia predefinita. Usa 'cvlc' per utilizzare vlc senza interfaccia.
libva info: VA-API version 0.40.0
libva info: va_getDriverName() returns 0
libva info: Trying to open /usr/lib/x86_64-linux-gnu/dri/i965_drv_video.so
libva info: Found init function __vaDriverInit_0_40
libva info: va_openDriver() returns 0
[00007f88b4c0d8b8] avcodec decoder: Using Intel i965 driver for Intel(R) Haswell Mobile - 1.8.3 for hardware decoding.
Failed to open VDPAU backend libvdpau_i965.so: impossibile aprire il file oggetto condiviso: File o directory non esistente
QObject::~QObject: Timers cannot be stopped from another thread

Selecting the option "Decoder VA-API throught X11"
$ vlc test.mkv 
VLC media player 2.2.6 Umbrella (revision 2.2.6-0-g1aae78981c)
[00005636cc1249d8] core libvlc: Esecuzione di vlc con l'interfaccia predefinita. Usa 'cvlc' per utilizzare vlc senza interfaccia.
libva info: VA-API version 0.40.0
libva info: va_getDriverName() returns 0
libva info: Trying to open /usr/lib/x86_64-linux-gnu/dri/i965_drv_video.so
libva info: Found init function __vaDriverInit_0_40
libva info: va_openDriver() returns 0
[00007fbd00c0d8b8] avcodec decoder: Using Intel i965 driver for Intel(R) Haswell Mobile - 1.8.3 for hardware decoding.
Failed to open VDPAU backend libvdpau_i965.so: impossibile aprire il file oggetto condiviso: File o directory non esistente

For other bigger files there is a noticeable difference in the CPU usage between the Xorg and Wayland session (Xorg lower).

I also attach vlcrc file from .config/vlc/