I tested the Voukoder connector 1.1.0 on Vegas Pro 17 with a project containing 5.1 channel audio.
The rendering of Voukoder is correct. Here Media info shows 6 channels in the final file rendered by Voukoder.
But Vegas Pro 17.0 has a bug in the So4compoundplug plug-in that causes AAC 5.1 audio channels to be imported into separate individual channels on the timeline.
If the So4compoundplug plug-in is disabled in the internal preferences, the old Compoundplug plug-in will be used and will import the AAC 5.1 audio channels correctly in the timeline.