I'm using Vegas Movie Studio 13 (Basic)
After editing my mp4 GoPro video, I noticed my audio track was different. Quieter.
It was not the identical copied audio.
AVIDemux gives an option to copy the audio and not render it again.
Does Vegas have this option?
Check to make sure that you don't have any audio FX on the track and that the track's volume is at 0dB FS. Vegas should not be doing anything to the audio unless you tell it to.
As far as copying the audio unaffected, I really don't know how Vegas handles this internally. If your source and target are the same format and you didn't add any FX you would think that it would just copy the audio but I've never asked nor read about how this is done internally.