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that's not what variable bitrate does. The videos where quality deteriorates have too low bitrate (constant or variable). If you use variable bitrate with the same overall bitrate, what will happen (If I understand correctly):
- first pass, encoder encodes with constant specified bitrate
- second pass, encoder checks the stats of encoded video and tries to get as close as possible to the sequence of images with the first pass (or original sequence of image). If it succeeds with a lower bitrate, then it keeps that low bitrate for the current image sequence. If it doesn't it increases bitrate and uses specified bitrate.
So you can only win in size, because of this image comparison feedback that prevents the software from lowering quality when more data is needed.
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