Vegas Pro will automatically do a 5.1 to stereo downmix if you use 5.1 audio in a stereo project or if you render a 5.1 project to a stereo file.
Normalizing according to EBU R.128 can be done by using the Loudness meter which is to be found at View/Window/Loudness Meter. But there is no plug-in which would do this automatically.
Also Vegas Pro can output a loudness log file for the current project by using Tools/Generate Loudness Log.
Sound Forge Pro 11, 12, 13 has a LUFS metering function as well, however it can render a loudness log with the "Statistics" tool .. much faster than real time. It cannot however auto set LU levels.
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.