The easiest way to know if your videos are playing with GPU acceleration are to watch the process of xplayer, mpv or whatever you are playing. The CPU usage should be no more than 10% for that process/program if it is using acceleration.
Let's manually play with vdpau to make sure it works before we make it permanent:
First make sure you have libvdpau installed:
sudo apt install vdpau-driver-all
If yo........