The following is from the CDex manual:
VBR Method Setting
The VBR method setting allows you the change the VBR algorithm which is used for the encoding (detailed information can be found in the LAME user documentation - see
http://www.sulaco.org/mp3/doc/html/index.html). The following selections are available
Disabled:
Don't use VBR; instead encodie with a Constant Bit Rate (CBR)
VBR-Default:
Use the default VBR method (currently set to VBR-MTRH)
VBR-Old:
LAME's first functional approach, based on masking, bisection in the bit domain.
VBR-New:
LAME's second approach, based on masking and direct noise allocation.
VBR-MTRH:
a combination of old and new (VBR) routines
VBR-ABR:
The Average Bit Rate (ABR) setting, the encoding principle is similar to what AAC uses as VBR encoding, it is based on perceptual entropy, but more like CBR than VBR. When you select the ABR option in the VBR Settings box, the ABR edit box will be enabled. In this edit box you can specify the target average bit rate. Of course, a larger bit rate will yield generally better-sounding (but larger) MP3 files.