[citizenkeith]

I finally got this working. The problem I reported/vented about in my above post was User Error (I had the vocal FX send misrouted).

Here's what I did to get it working:

1) Create a new track. I called it Music Buss.

2) You are going to route all your tracks, except the vocal and vocal sends, to this Music Buss track. For each track sent to the buss, go to I/O section and uncheck "Master Send", then Add New Send and choose the Music Buss. Alternatively, you can do this in the Routing Matrix.
NOTE: If you are using Folders, be sure to uncheck "Master Send" only on the parent folder, not the children tracks within it.

3). Go to the vocal track I/O and change Track Channels from 2 to 4.

4) Go to the Music Buss, Add a New Send and choose the vocal track. Adjust the output for this send to read "Audio: 1/2 => 3/4". See attached image for details.

5) Now the Vocal Rider is receiving music via the sidechain. Make your adjustments accordingly.

6) You can use Latch mode to write the volume changes as automation. In Vocal Rider, choose "Write" on the bottom of the plugin. Right-click on Track Envelopes/Automation section (it's "Trim" by default) and choose "Latch (Record fader movements after first movement)" from the menu. Hit play and watch the envelopes write themselves. Switch Vocal Rider to Read mode and you're done. Switch Latch mode back to Trim mode. Sometimes I'll make adjustments to the automation, especially if there are some vocal breaths that get too loud.

Hope that helps!