The core library and the included plugins (minus ocr) can be cross-compiled pretty easily. You pass
Code:
--host=x86_64-w64-mingw32
to configure.
The Python module is the annoying part, because you must also cross-compile Python, and I think the Python developers don't make it easy.
But this is all irrelevant, because VapourSynth has a C API, so you can just use the official binaries compiled with MSVC. You don't have to cross-compile it for ffmpeg.
Correction: VapourSynth has a
reasonable C API, unlike Python.