Comment # 30 on bug 1139791 from Wolfgang Bauer

(In reply to Stakanov Schufter from comment #29)
> va-api with vlc works for vlc, but kaffeine and vidcutter seem to default to
> vdpau? Even when setting vlc to use va-api, kaffeine does not honor this and
> crashes.
Kaffeine does not use VLC's settings, no.
It is a separate application, it only uses libvlc for the actual media
playback.

You can tell kaffeine to pass certain options to libvlc though, see the
included /usr/share/doc/packages/kaffeine/README.md (sections "Changing the
libVLC output plugin" and "Setting VDPAU acceleration").

And quoting from there to answer the question:
> By default, libVlc will try to use vdpau hardware acceleration in order to
> decode the video stream at GPU.