Oh, that awful, misleading thread. The ATSC A/52A (AC-3) standard (refer to sections 6.1.12 & 7.8, and also this post by pandy) provides detailed info about how proper downmixing should occur for AC-3.
Note that the Low Frequency Effects (LFE) channel may be omitted when downmixing, and I believe that's what
Changing default downmix method
If you want to change the downmix method then try the suggestion by pandy in the same thread:
I did not test this, so I'm not sure it will do what you want.
Also, investigate the
-dmix_mode private option. See
ffmpeg -h encoder=ac3.
-vbr option is used as a "private" encoding option for some encoders, not as a global option:
Private options are specific to the given container, device or codec.
These options are listed under their corresponding containers/devices/codecs.
...meaning the use of this option and what it does may vary depending on the encoder; if the encoder even supports this option (refer to
ffmpeg -h encoder=<encoder name>). Other audio encoders may use
-q:a) for VBR.
ac3 ignores both