Does the vendor kernel support more of the board's peripherals than the mainline kernel?
Had that issue with some Odroid boards, where the vendor kernel supported MFC hardware acceleration but the vanilla kernel didn't/doesn't. I'd like to avoid that
Yes, that’s the main reason to run the vendor kernel. Mainline support is improving all the time though.
I believe I needed the vendor kernel to use video through the USB-C port, and to use the HW acceleration for transcoding in Jellyfin. This situation may have changed since my last attempts.