ffmpeg encode vp9

Variable Bitrate

libvpx-vp9 offers a variable bitrate mode by default. In this mode, it will simply try to reach the specified bit rate on average, e.g. 1 MBit/s. This is the "target bitrate".

ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 1M -c:a libvorbis output.webm

Choose a higher bit rate if you want better quality. Note that you shouldn't leave out the -b:v option as the default settings will produce mediocre quality output.

REF: https://trac.ffmpeg.org/wiki/Encode/VP9