http://www.rarewares.org/ogg-oggenc.php
I see Oggenc2.84 using libVorbis v1.2.0 and Oggenc2.84 using aoTuVb5.
what's the different are libvorbis and aoTuVb?

libVorbis 1.2.0 is the "official" release from Xiph.org.

aoTuVb5 is a modified version which aims to improve performance. Many people here report it produces better results when encoding at low quality settings (q<=1.0).

See the wiki:
HA Wiki: AoTuV

I wondered about the differences between the two... but never looked myself.

About the aoTuVb5(aoTuV beta5), please look at this page.

Excepting Rockbox as an aftermarket upgrade, are there compatibility issues with portable music players (that natively support Vorbis) if using aoTuV builds over "official" Xiph ones? I am not encoding my stuff at the ultra-low rates (usually quality between 4 and 7), anyway.

If I am not mistaken, there are no compatibility issues between them.
My Iriver H120 plays AoTuV encoded Vorbis files just fine both with Rockbox and also with the original firmware. (I do not use very low bitrates, either. -q 4,5 and -q 5)

I noticed that those wiki's haven't been updated in awhile as they don't include the libVorbis 1.2.0 release.

I may be misinformed, but libVorbis 1.2.0 is now newer then aoTuVb5. While you can download version of oggenc using libVorbis with the aoTuVb5 tunings patched in, which is actually better now, the default tunings(newer) or the aoTuVb5 tunings? Anyone done any tests?

Since libvorbis 1.2.0 did not contain any quality improvements - see changelog at http://svn.xiph.org/trunk/vorbis/CHANGES - its release did not change the situation regarding which encoder is better (or best). Therefore aoTuV beta 5 is still to be regarded as best (producing the best quality) encoder.

Ah interesting. May they will finally incorporate aoTuV tunings in the next official build. Vorbis development seems to be extremely slow, which could be a sign of stability, or a lack of interested developers. It is too bad, with a more active team it could be alot bigger then it is now. Whereas Lame, on the other hand, seems more active in comparison.

Quality, not performance. The Lancer builds are performance driven.

So: aoTuV = aims at quality, Lancer = aims at speed with possible quality loss.