On GeForce 9400, mpv works with both VA-API and VDPAU. This is on GNOME/Xorg, I
can't checkout GNOME/Wayland on this machine.

Linux 5.2.8-arch1-1-ARCH
local/libva-mesa-driver 19.1.4-1
local/mesa 19.1.4-1
local/mesa-vdpau 19.1.4-1

$ lspci | grep VGA
04:00.0 VGA compatible controller: NVIDIA Corporation C79 [GeForce 9400] (rev
b1)

$ mpv --hwdec=vaapi -vo=vaapi /path/to/MP4
Playing: /path/to/MP4
 (+) Video --vid=1 (*) (h264 1280x720 23.976fps)
 (+) Audio --aid=1 --alang=eng (*) (aac 2ch 44100Hz)
Using hardware decoding (vaapi).
AO: [pulse] 44100Hz stereo 2ch float
VO: [vaapi] 1280x720 vaapi[nv12]
AV: 00:00:24 / 00:02:03 (20%) A-V:  0.000

$ mpv --hwdec=vdpau -vo=vdpau /path/to/MP4
Playing: /path/to/MP4
 (+) Video --vid=1 (*) (h264 1280x720 23.976fps)
 (+) Audio --aid=1 --alang=eng (*) (aac 2ch 44100Hz)
Using hardware decoding (vdpau).
AO: [pulse] 44100Hz stereo 2ch float
VO: [vdpau] 1280x720 vdpau[yuv420p]
[vo/vdpau] Compositing window manager detected. Assuming timing info is
inaccurate.
AV: 00:00:16 / 00:02:03 (13%) A-V:  0.000