I use Nero to encode OOG files from shn and FLAC files that I download. (I'm sure that there are better products, but Nero is what I use).

There are only a handful of settings available to me in Nero's version. I can choose Constant Rate and select a bitrate (45-500), or Variable (range 0-100).

I play the files back on my Rio Karma DAP.

What are the recommended settings? I would guess VBR@ 100 should be best. Is there a substantial disk usage pendalty if increasing from 50 to 100, or choosing this over a constant 192 bitrate?

Vorbis is designed to be used primarily as a variable bit rate encoder. Most front ends for it use a qscale to assign quality to the encoding. -q5 and above (to -q10) are considered the higher quality settings. Portables that can play back vorbis files sometimes have issues with larger (-q7 and above) files, or very small files (-q-1 or -q0) so testing is needed for compatibility. The nero "system" sounds like it will make -q5 files using their 0-100 scale at 50 setting, which should be ok for most portables that support vorbis. Some vorbis expert will now step in and explain why you should avoid cbr with vorbis

Wouldn't it be the same reason you wouldn't want to make CBR files in any format? You either overshoot or undershoot the bitrate for each frame. Actually I don't even think Vorbis has a real CBR mode, just really really strict ABR.

This is the definition of CBR for any audio format: a really really strict ABR.
Or, ABR mode is a CBR with unlimited reservoir size.