Are all Blender's audio encoding choices VBR?

I’ve been trying out various video and audio encoding options, trying to determine the optimum to be used on my show reel video. So far, while testing only Blender’s output options, I’ve encoded the audio using mp3, AAC and AC3, and since I’m using MPEG as the over-arching compression tool suite, it’s all going into the .avi container format.

I try these outputs on various Windows-platform media players I have on board to see where problems might arise in terms of providing a widely-playable format. Not all seem to work well, but one consistent aspect I’ve found is when loading the videos into Virtual Dub, the utility always reports that the audio is variable bit rate (VBR), which it says is not a good choice for the .avi container.

The audio codec specs in Blender seem rudimentary, with only bitrate (apparently fixed, not variable) and volume (gain) being editable parameters, regardless of the codec choice.

Why, then, is the audio apparently tagged as VBR? This wouldn’t be much of a question except that in some of the output configurations, the a/v synch goes way out (AAC in particular) or the vid glitches in other small ways, like starting a second or two past the beginning of the vid.

I want to find encoding schemes that are as stable as possible on a majority of platforms using the most common players and still deliver decent image and sound quality, so any info on Blender’s choices and how they perform would be much appreciated.