Possible fixes:

Either make vlc require libvpx-1.7 instead of libvpx-1.8 with no other changes.

Or patch vlc to not use the newly removed constants.  Reading the vlc commit log,
they say that they removed those because they were unused anyway--so it shouldn't
have a negative impact.

I've posted the latter patch to vlc-devel.

I've done the former to guix master for now.