Hi Olaf,
there is no special option for Vorbis Comments because these are the only valid tags for FLAC and Vorbis. To remove them, just choose 
, File > Remove tag or [Ctrl+R].
If you want to remove ID3v2 and leave the Vorbis Comments, just remove the tags and choose Edit > Undo or 
to rewrite the Vorbis Comments without the ID3v2 tags.
Best regards,
Florian