> "Could not initialize VapourSynth scripting."
It always means some mess in Python configuration - "vapoursynth.so" is missing from site-packages or built for another Python version.
The correct config:
bash-3.2$ ls -l /usr/local/opt/python
lrwxr-xr-x 1 chainik admin 22 Apr 18 22:38 /usr/local/opt/python -> ../Cellar/python/3.6.5
bash-3.2$ otool -L /usr/local/bin/mpv
/usr/local/bin/mpv:
...
/usr/local/opt/vapoursynth/lib/libvapoursynth-script.0.dylib (compatibility version 1.0.0, current version 1.0.0)
/usr/local/opt/vapoursynth/lib/libvapoursynth.dylib (compatibility version 0.0.0, current version 0.0.0)
bash-3.2$ ls -l /usr/local/opt/vapoursynth
lrwxr-xr-x 1 chainik admin 26 Apr 18 22:38 /usr/local/opt/vapoursynth -> ../Cellar/vapoursynth/43_1
bash-3.2$ ls -l /usr/local/lib/python3.6/site-packages/vapoursynth.so
lrwxr-xr-x 1 chainik admin 75 Apr 18 22:38 /usr/local/lib/python3.6/site-packages/vapoursynth.so -> ../../../Cellar/vapoursynth/43_1/lib/python3.6/site-packages/vapoursynth.so
bash-3.2$ otool -L /usr/local/opt/vapoursynth/lib/libvapoursynth-script.0.dylib
/usr/local/opt/vapoursynth/lib/libvapoursynth-script.0.dylib:
...
/usr/local/opt/python/Frameworks/Python.framework/Versions/3.6/Python (compatibility version 3.6.0, current version 3.6.0)