After messing about with the feedback routing, I found it to be really inconsistent and buggy. That, or I'm missing something.
Either way, there's another method to route individual MIDI tracks from Ensemble instances to their separate audio tracks (and thus be able to freeze them as stems). Right click on the Ensemble plugin in the FX chain window and select "Create multichannel stereo outs" (or something like that). It will automatically create as many properly routed stereo out tracks as the VEP instance has set in its configuration window. Then, route each channel in VEP7 to the proper out (1/2, 3/4, etc) and the midi will send audio to *that* track. It increases track count but it seems more reliable to me than the feedback routing method.