SUMMARY
Hi all! Here follows a bug report in conjunction with Kaffeine 2.0.18. Unfortunately Kaffeine does NOT respect additional entries in its "Settings" -> "libVLC arguments".

In my case, VLC only works properly with the arguments --vout=gles2 and --avcodec-hw=vaapi. Therefore Kaffeine should also use this settings.

STEPS TO REPRODUCE
1. Start up Kaffeine
2. Go to "Settings" and then "libVLC arguments"
3. The additional arguments shows no effect

OBSERVED RESULT
Kaffeine permanently ignores and additional libVLC arguments, it always falls back to VDPAU with message: 
"Using G3DVL VDPAU Driver Shared Library version 1.0 for hardware decoding"

EXPECTED RESULT
The --vout=gles2 and --avcodec-hw=vaapi vlc parameters should be accepted. Kaffeine should start up and use the OpenGL ES 2.0 video output and the VA-API hardware decoder.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kubuntu / Ubuntu 20.04.3 LTS (Focal Fossa), kernel 5.11.0-40-generic x86_64 bits
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8

ADDITIONAL INFORMATION
Used drivers: Mesa 21.0.3, Mesa 22.0.0 devel

Note, the mpv Media Player starts right out of the box without any additional settings and performs awesome reliable. So maybe it would make sense to chose that on instead of the libVLC library. ;-)

A similar bug was filled in 2019 at the Gentoo Bugzilla: https://bugs.gentoo.org/686646