Please try this and report your findings.

Try this A/B test.

Set up your project for 5.1 audio; put video on one track, and some loud audio on another.
Adjust the audio levels using audio envelopes, and -6db hard limiters; even move the volume slider. Get the sound to a db level you like.

Now render the video and audio to a new track.
When it’s done rendering; solo the new rendered tracks.

Now listen to the new rendered track..... it will be loud, like you never adjusted it before the render.

Listen to the original audio track solo, and it will sound like you wanted it.

I don’t recall this problem with Vegas 4.
Anyone have any thoughts?