When I try to run the VLC Media Player in Full Screen mode I get just a black screen or in the best case single frames of the video displayed. Same problem with any mp4 video that I have in my collection. The video shows normal when not in fullscreen mode.
I have the Pi-Top Touchscreen Display connected via the green Display cable.
VLC Media player is latest version and the pi-top OS has all updates installed.
Martin Klein
Found a similar thread here about this issue.
Can you try:
sudo apt update
sudo dist-upgrade -y
Also, please try using a micro HDMI cable plugged into the micro HDMI port on the pi-top [4] and see if the behaviour changes
also there’s a thread here with some other suggestions. Seems there are many discussions of this problem across google 
Try this in vlc
Tools->Preferences->Video->Output->OpenGL for Embedded Systems 2 video output
Many thanks ! That fixed it ! Great Support ! Wow !
Kind Regards
Martin Klein