--demuxer-lavf-format=vapoursynth (or vapoursynth_alt). The 64-bit mpv from the package I posted on the 10th also supports using VapourSynth as a video filter (for say, 24->60fps motion interpolation).
.vpy can associated with the above command in the config file, so you don't have to remember to use it:
Code:
[extension.vpy]
demuxer-lavf-format=vapoursynth