1. Prefers to use VapourSource for vpy input, as it is a native VapourSynth source filter. Put VapourSource.dll in AviSynth’s auto-load folder, or else x264 would try AVISource/HBVFWSource as before. Keep in mind that unlike HBVFWSource which will automatically imply –input-depth 16, with VSImport x264 wouldn’t know if the vpy output clip is 8-bit or not, so it is needed to manually set
--input-depth depth if depth is not 8.
2. Prefers to use LSMASHVideoSource/LWLibavVideoSource as source filter than FFMS2 for many input formats, e.g., mp4/mov/mkv/m2ts/…. Still able to fall back to FFMS2 if LSMASH/LWL fails.