Here is the short term help.
There are three libvpx's in the repo...
libvpx version 0.9.1
libvpx version 0.9.6
libvpx version 1.1.0
Avidemux in the repo was built against libvpx version 0.9.1, so the quick fix is to just use gslapt to down grade to that version.
However, the right fix is for me to build a new Avidemux built against the newest version of libvpx and make that available.
I'm working on that now.