It won't work with Vegas, as it's not VfW. There are third party free tools to encode in webm, e.g. SUPER. Then, VLC is the best way to decode webm to test it. If you want to edit first, export in an intermediate codec, like Huffyuv, so SUPER/ffmpeg can read it. Ffmpeg will also read Avid DNxHD.
You're talking about AVI VP8, not webm. It's not for sure that every HTML5 browser with VP8 support also supports the format in the AVI container. The standard/suggested one is .webm, that's the one Vegas doesn't support.