I'm not sure digital audio can directly route to the vocoder. Take a look at the Common Audio edit menu

1) Press [PERFORMANCE] (HOME)
2) Touchscreen (TS) - press on the performance name (big text near top of of screen)
3) TS: (left hand side) choose "Edit"
4) TS: Menu "Audio In" -> "Mixing" notice the 2nd half of the screen is dedicated to "Digital In". There is an "Output Select" (lower right) which is associated with the digital input. You can route this to Assign L&R (if stereo is wanted) or just one of the two channels Assign L or Assign R
5) Route, with a physical cable, the ouput of Assign L&R (or one channel) to your A/D input
6) Use A/D input as normal when applied to vocoder modulator.

Also, A/D has a routing option - you likely want to route A/D to "OFF" to prevent from hearing the A/D get passed through to Main L&R.

"Direct Monitor" also plays a part - so check this is off if you still hear unwanted audio.

I sort of wish, for variety, we could use digital audio channels as sources for FM-X as well as directly tie digital channels to the vocoder. Using Assign L&R (and external cabling) instead is a reasonable alternative to internal routing. Internal routing does eat up resources.

Jason, this is brilliant, thanks. I'll test the analog loopback method as you've suggested. My goal is to be able to use my boom-mounted microphone through the DAW + processing, without needing a separate headset mic for the vocoder.

BTW: I really like the series of practical videos by Michael Tan. The vocoder video is here:

https://youtu.be/pcLIl304dIk?t=68

and I've skipped passed the theoretical portion - but you can review the entire video if you want. Just replace the Microphone with a line-in coming from your Assignable Output(s).