This means that there is no ACM codec on your system that can perform the requested conversion. You can use the NAudioDemo app that comes with NAudio to examine all the ACM codecs you have installed on your system and their supported input and output formats. Windows certainly doesn't come with a Vorbis ACM codec, which is probably why your code doesn't work. Even if you had installed a Vorbis ACM codec, you need to get the WaveFormat exactly right or you will get the ACM not possible error.
You'd probably be better off trying to use the the NAudio support that comes with NVorbis in any case.