Hello,
I've had this issue with Vegas Pro 15, and also 18 (I just upgraded and have the same problem), where before I used to be able to drag a video that has two audio tracks into Vegas and it would automatically add both tracks, so I could modify them freely. This has stopped working recently and I have no idea why; it only detects a single audio track and I can seemingly no longer edit both at the same time.
I'm not sure what changed to cause this, but other video players (like VLC) can still correctly identify and swap between the audio tracks, but Vegas doesn't seem to see them anymore. For reference I am recording gameplay footage via nVidia Shadowplay with separate audio tracks enabled between the microphone and the speaker audio.
Does anyone know how to fix this? I would like Vegas to behave like it originally did where both tracks were correctly identified and added into the timeline once a video is dragged in automatically.