I am unaware of this in Vegas.
I know you can ask Vegas to attach timeline envelope nodes to a clip but I have seen these become detached under certain circumstances. It would be nice to have the same envelope automation on the volume line of the clip itself.
You can pull the volume down within the clip (and I wish you could boost it as well without having to normalize). I've never had a problem with envelope nodes detaching wihtout turning that option off.