The build in GPU "AMD Radeon Vega 8" has the ability to accelerate videos if you have the following packages installed

vdpau-driver
vdpau-video
libvdpau-va-gl1
libvdpau1
mesa-vdpau-drivers
vdpau-driver-all
vdpau-va-all
for testing helpful packages are
vdpauinfo
vainfo

Unfortunately vlc relies on environment variables to determine the hardware acceleration possibilities of the GPU and defaults to "nvidia".

To "convince" vlc to use the build in video decoding acceleration of the AMD GPU you add the following lines to /etc/environment

sudo echo -e "export LIBVA_DRIVER_NAME=radeonsi\r\nexport VDPAU_DRIVER=radeonsi" >> /etc/environment

should give you the following contence of  /etc/environment.

$ cat /etc/environment
export LIBVA_DRIVER_NAME=radeonsi
export VDPAU_DRIVER=radeonsi

After logout and logging in again, you can check in the terminal with

$ set | grep radeon
LIBVA_DRIVER_NAME=radeonsi
VDPAU_DRIVER=radeonsi

for success.
vlc will now run smooth for all local users.

A sample output on the terminal would look like this

$ vlc
VLC media player 3.0.8 Vetinari (revision 3.0.8-0-gf350b6b5a7)
libva info: VA-API version 0.39.4
libva info: va_getDriverName() returns -1
libva info: User requested driver 'radeonsi'
libva info: Trying to open /usr/lib/x86_64-linux-gnu/dri/radeonsi_drv_video.so
libva info: Found init function __vaDriverInit_0_39
libva info: va_openDriver() returns 0

In this mini howto I only tested my laptop with 2016 HW  "AMD Radeon Vega 8" running on a backported 4.19.0-0.bpo.1-amd64 kernel with mesa-va-drivers and mesa-vdpau-drivers (version 18.2.8 ...).
If you have Intel GPUs you might need "LIBVA_DRIVER_NAME=iHD" or "LIBVA_DRIVER_NAME=i965" or ...
Also "VDPAU_DRIVER=va_gl" might be a good choice - or not.
You have to find out the combinations yourself.

Testing with your hardware is easy.
1) Make sure you have hardware acceleration turned on.
run

glxinfo | grep Mesa 

in a terminal and if it outputs something like

...
client glx vendor string: Mesa Project and SGI
OpenGL core profile version string: 4.5 (Core Profile) Mesa 18.2.8
OpenGL version string: 4.4 (Compatibility Profile) Mesa 18.2.8
OpenGL ES profile version string: OpenGL ES 3.2 Mesa 18.2.8

your  acceleration is turned on.

2) check for vdpau and va packages on your system.
e.g. by

$ dpkg --get-selections | grep vdpau

3) set your environment variables in a terminal

$ export LIBVA_DRIVER_NAME=radeonsi
$ export VDPAU_DRIVER=radeonsi

4) run vlc in terminal

$ vlc

and open a video file

5) check for error messages of vlc in the terminal

.....
Failed to open VDPAU backend libvdpau_nvidia.so:...
.....

6) goto 3) and change the variables...

Good luck!

Jan