Use ffmpeg instead (32-bit version, if you want unbroken avisynth support):
ffmpeg.exe -i "input.mp4" -an -c:v libvpx -b:v 0.5M "out.webm"
This encodes the video only, doesn't keep audio. Change 0.5 to whatever you want to increase/reduce file size (and quality). VLC doesn't have features to encode videos to webm, or at least last time I used it in the past decade it didn't have any.