yes, when trying to “pamac build ffmpeg-amd-full-git” i am also seeing same messages about unable to satisfy dependency, vapoursynth was not installed and installing it not help
You need to first build the package and then install it, AUR helpers are unsuitable for this task
For an optimal workflow is better to build ffmpeg-amd-full-git and its dependencies with chrootbuild (from manjaro-chrootbuild) and making your own local repository with a static webserver like darkhttpd