After some digging around, I found this:
http://www.db-audioware.com/dbd.htm
It's compressor/expander with a side-chain function that works great as a compressor/expander/ducker. While the author claims the sidechain has too great a latency in Acid, I've tried it in Vegas 4 and it works really well.
You use one on the track you want to send the control signal from (with no processing if desired), and it gives you 6 chain channels to direct to other instances. You can have one source signal control multiple destinations. You can control a track from another track, a bus from a track or even a track from a bus. The response time is great.
21-day full trial on the demo, so you can really try it out ($39 to purchase). Other than the TC Native bundle costing a lot more, I don't know of any other way to do this kind of sidechaining. If any of you do, please let me know.
This has made my work a whole lot easier: a few less volume envelopes to draw...I'm going to check out the other plugs.
Angels
PS1: I don't work for them, or have anything to do with them! I haven't even actually bought the plugs yet.
PS2: Foundry should seriously consider licencing this gentleman's technology and incorporating it into their next upgrade package; it's sorely lacking in Vegas 4, and people have been requesting it for a long time.