The request is about VBR.As the encoder in VAAPI driver is improved and new feature VBR is added for AVC/VP8/HEVC. Now upstreaming is on-going. here VP9 is ready in driver. The middleware should support this.
Thanks for your bug report, but GStreamer tracks bugs in GNOME bugzilla at
Please file any bugs or patches there. We use the Freedesktop bugzilla only for account reuqests.