>=libvpx-1.8.0 globally masked you could disable vpx use flag globally in /etc/portage/make.conf to eliminate any need for libvpx.
I need vpx for ffmpeg and it was preventing the x265 update.
I comment out the mask which is temporary which allowed the update to proceed.
rj@funtoo /var/git/meta-repo/kits/core-kit/profiles/package.mask $ sudo nano funtoo-cautionary
# Allow firefox to merge cleanly
You could also add the libvpx-1.7.0.ebuild from the 1.3-release branch to a local overlay.