FFmpeg - Encode VP8

In addition to the "default" VBR mode, there's a constant quality mode (like in the x264 encoder) that will ensure that every frame gets the number of bits it deserves to achieve a certain quality level, rather than forcing the stream to have an average bit rate. This results in better overall quality and should be your method of choice when you encode video with libvpx. In this case, the target bitrate becomes the maximum allowed bitrate. You enable the constant quality mode with the CRF parameter:

ffmpeg -i <input.file> -c:v libvpx -crf 8 -b:v 0 <output.file>

Reference: [FFmpeg: VP8]
Reference: [superuser: VP8 single pass conversion with good quality in ffmpeg]