There are fairly mainstream devices with decent Vulkan support but poor hardware decode coverage for the codecs people actually get on the web. Polaris era Radeons have H.264 and HEVC decode, but VP9 support is absent (or not exposed in many common Linux paths) so YouTube is sloppy. The Raspberry Pi 5 is another example: it has hardware HEVC decode, but YouTube 4K is generally VP9 or AV1 rather than HEVC, and Pi 5 does not advertise VP9 hardware decode.

I think Vulkan Video is just another api to access those hardware decoders. It's not going to bring support for codecs to hardware without the support.

Yea, I'm most-hopeful for some of my lowest-end devices. Those as-cheap-as-possible CPUs tend to have a very strange set of accelerators for codecs.