In reply to:

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I think this is how lame nogap works - or doesnt work. It encodes the whole cd as one stream and then breaks it into parts.

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It does some other stuff, too - like ensuring that the bit reservoir is flushed between tracks (to prevent dependencies between frames in different files).

None of which affects the fact that we ought to have [em]exactly the same MP3 frames[/em] in the stream with or without the VBR header, AFAICS - so the VBR header should make no difference at all to the sound output in normal (1x) playback.
Anyway, I've worked-around by re-encoding at 160kbs CBR for those tracks.