You could cheat it by spliting the clip around the area you want to adjust differently, but it will still be a blanket adjustment. The track envelope is probably the best way to go...and the edit points will move with the clip.
You can't assign individual clips to busses -- only tracks. If you have some specialized thing you're trying to do to one specific clip, you might want to just put that clip on a separate track. You have unlimited tracks in Vegas, so its not like you're wasting anything.
I know whether it's audio or video, if I have some specific effect I want, I generally set up s special track for it. Adds to the scrolling, but certainly makes it easier if you want to move things around...
If you want to control more than one audio track at one single time, while still having yet a third independent audio track, then buses are for you. But if it's 2 or less tracks, and you don't have secondary hardware routing, buses don't do a lot for you. For me, the rule is 3 or more get a bus.