I tried libvorbis encoder by ffmpeg for converting some FLAC file to Vorbis. I found out that if you use both -aq and -ab with it, the encoding is done using the -aq setting, which sets bitrate accordingly. I suppose it uses this table:
http://en.wikipedia.org/wiki/Vorbis#Technical_details (though sizes of files encoded using either -aq 8 or -ab 256k did not match exactly, but were relatively close.)
If you use -aq setting with libvorbis, there is no big difference in encoding times, when compared to vorbis. But if you use -ab setting, encoding time more than doubles. I don't see any advantage in using the -ab setting.
I tried playing the converted files by alsaplayer and VLC. ffmpeg doesn't seem to transfer audio tags, such as author and title
VLC can read more metadata from the files encoded by libvorbis. VLC gives very different results for the track duration: It can not give track duration for the file encoded by vorbis, gives 3 m 35 s for the one encoded by libvorbis using the aq argument, gives 3 m 33 s for the one encoded by libvorbis using the ab argument, and for the original flac it gives 3 m 16 s. In alsaplayer there are less differences: the duration of the file encoded by libvorbis is 3 m 15 s whereas the original and the one encoded by vorbis are 3 m 16 s, which could be a rounding error, I guess.
Finally, here are the versions of the programs used:
FFmpeg version SVN-r19352-4:0.5+svn20090706-2ubuntu2, Copyright (c) 2000-2009 Fabrice Bellard, et al.
configuration: --extra-version=4:0.5+svn20090706-2ubuntu2 --prefix=/usr --enable-avfilter --enable-avfilter-lavf --enable-vdpau --enable-bzlib --enable-libgsm --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-libvorbis --enable-pthreads --enable-zlib --disable-stripping --disable-vhook --enable-gpl --enable-postproc --enable-swscale --enable-x11grab --enable-libdc1394 --extra-cflags=-I/build/buildd/ffmpeg-0.5+svn20090706/debian/include --enable-shared --disable-static
libavutil 49.15. 0 / 49.15. 0
libavcodec 52.20. 0 / 52.20. 0
libavformat 52.31. 0 / 52.31. 0
libavdevice 52. 1. 0 / 52. 1. 0
libavfilter 0. 4. 0 / 0. 4. 0
libswscale 0. 7. 1 / 0. 7. 1
libpostproc 51. 2. 0 / 51. 2. 0
built on Oct 13 2009 22:15:16, gcc: 4.4.1
VLC media player 1.0.2 Goldeneye
alsaplayer 0.99.80
I don't know if this is relevant, but the libvorbisenc2 package version is 1.2.0.dfsg-6ubuntu0.1 here in Xubuntu 9.10.