Blu-ray Rip - Best Audio Encoding

[LionGod8]

Hello everyone.

I make low bitrate (4,000-6,000 kbps) 1080p Blu-ray rips
and need your opinion about audio encoding - the best way to keep most quality.

In short:
When I rip a Blu-ray and want to encode the audio:
1. In case of only DTS audio track on Blu-ray - what is better, to encode to 640 kbps CBR AC3 or to VBR with another encoder (like AAC or Vorbis or maybe something else) ?
2. In case Blu-ray has one lossless DTS track and one 640 kbps AC3 track - what's better, to keep the 640 kbps CBR AC3 track as is or to encode to VBR (with another encoder) from the DTS lossless track ?
3. Is there a way of keeping all 7.1 channels when encoding to AC3 ?

Thanks in advance.