I'm gonna guess here and say, yes it will. By how much depends on how efficent the battery life is on your player. Of course, each frame is always decoded independently ANYWAY. And, because VBR not only goes up, but sometimes down, it may actually stay the same or save you some battery life - it probably wouldn't be anything noticable, though.
Anyway. For example, if you were to ever watch a LAME encoder window while it's encoding a VBR mp3, it displays a graph of 128 to 320kbps and marks how often those bitrates occur in the stream by a line of X's displayed next to it.
If you watch a VBR mp3 being played in Winamp, it dips down to 128 and 160 kbps a lot. For more complex frames, it'll go to 320, but not for long. So... I'm gonna have to guess that because a VBR mp3 switches to lower bitrates so often in the stream, it'll make up for the high quality bitrates that burst into the stream during various points of the song as well. This is just a guess. VBR mp3's might not affect battery life at all, because like I said each frame is decoded, just really fast... so... it might not matter.
If it does affect battery life, I can't imagine it would be by much. MP3 decoders are pretty efficent.
