Vorbis is a outdated free as in freedom lossy audio codec. It was a good alternative to the then patent-riddled MP3 format (The MP3 patents have since expired). Vorbis audio was typically stored in Ogg file containers and audio files in Vorbis were typically referred to as "Ogg Vorbis".
Vorbis has since been replaced by the better and also free Opus audio format. You do NOT want to create new Vorbis files. Keep the ones you have which can't be acquired as FLAC but do not make new ones.
HOWTO convert MP3 MP3 to OGG
Converting from one lossy format to another lossy format is never a good idea. You could do it with a combination of mpg321 and vorbis-tools using this command:
mpg321 input.mp3 -w - | oggenc -o output.ogg -
..but you DO NOT WANT TO DO THIS. NO. One does NOT convert from one lossy audio format to another.