Pomelo Free MP3 2 Vorbis is free, of course. It's professionally designed for converting MP3 to Vorbis easily and fast. The interface is user-friendly and output quality is not bad. So just give it a try.
E0E Free MP3 to Vorbis Converter SE is an easy to use and professional conversion program which allows you to convert MP3 to Vorbis format totally free. The conversion speed is fast and output quality is great.