In theory, yes, the quality will be reduced. In reality, the human ear probably can't hear any difference.
reply to TomBrooklyn
Yes it will be reduced, but the question is how much, and for reasonable rates, the difference may very well be imperceptible.
A better question would be that for 2 files of the same size, both created from the sane original material, but one encoded with CBR and the other with VBR, which will have a higher audio quality? Chances are good that the VBR will be better, although the difference may not be much either.
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