PMB, that really depends on the program.

Technically speaking, the VBR bit-rate you choose is supposed to be the guaranteed minimum; however, it's rather ambiguous whether some programs use standard VBR when you choose VBR, or a type of VBR called ABR ("average bitrate"), which would aim for an average bitrate centered on the bitrate you chose.

It's worth checking to see if your program makes a distinction between VBR and ABR.

By the way- a neat thing about VBR is that, when there's only absolute silence, the bitrate can drop down to 32k (even if you set a hard minimum). With 320k, you use 320k for silence, too.

RD