Vegas Pro downmixes 5.1 including normalization to Lt/Rt and with a rear channel ratio of 0,70710677 while you can modify this ratio via the internal preferences.
"- my understanding is there is still no solution of automaticaly make audio-ducking ( in a similar way like Video-Pro-X does it)"
Don't know how Video ProX does this. There is no side-chain processing in Vegas Pro which some compressors would use for auto-ducking. The most comfortable way to do this imho is using the Voice-over tool of Vegasaur.