Thanks everyone, I *really* appreciate the efforts to help.
The thing is, while the general principles are the same with other FX units, there are details specific to the VL2 which make it trickier.
rscain, TC Helicon just told me that VL2 and VL3 cannot be set up the same way for this, as VL3 cannot be used as a digital insert, it must be used in the analgoue realm. They wrote:
"You cannot use a VL3 as an insert effect in your DAW the same way that you can with a VL2.
VoiceLive 2 you can use as a 100% digital insert via USB or SPDIF, VoiceLive 3 doesn't have SPDIF and we've dropped the USB audio part, so integrating a VL3 with you DAW is more of an "analogue" process, where you can post-process audio by using the analogue ins and outs on the unit.
The short of it is: With VL2 you can use it as a digital insert effect. With VL3 you cannot."
Also, unlike other FX units, VL2 needs to be fed two signals:
A. one with the backing track, in order to let VL2, using key detection, know how to harmonize, and
B. the other, the dry vocal signal to be processed.
This requres some apparently very specific settings within VL2 so that signal A does not end up getting intermingled with signal B on the return journey.
Therefore -- not to be ungrateful to everyone for their help, on the contrary -- but I'm really hoping to hear from anyone who's gotten, specifcally, VL2 and Sonar X3e or Platinum to work this way.