As far as I understand, libvpx were added in jhbuild in times of OpenWebRTC. Nowadays, LibWebRTC is used and libvpx is installed through Tools/gtk/install-dependencies, thus it should be removed from jhbuild.modules for gstreamer.
Created attachment 389479 [details] Patch
Informal r+ from me :-)
Comment on attachment 389479 [details] Patch Clearing flags on attachment: 389479 Committed r255558: <https://trac.webkit.org/changeset/255558>
All reviewed patches have been landed. Closing bug.