I revisited the issue tonight with no improvement. The failure was a bit more obvious and reproducible.
SCENARIO 1: EXS to VE PRO Instance with Other Loaded VSL Instruments
I have an EXS channel strip with Inserted Vienna Ensemble Pro Audio Input and a bypassed I/0. It is received by a VE Pro Instance Audio Input channel.
Works. Sounds great. But when I hit play, the EXS signal is lost.
TO REGAIN SIGNAL, I just play something, anything, on the receiving VE PRO instance. Then I go back to the EXS instrument and it plays again.
Until I hit play in Logic again. Rinse, repeat.
Here's one diagnostic detail. If I play a different VE PRO Instance, one that has nothing to do the EXS / VE Pro link noted above, EXS playback does not return. I have to select the receiving VE PRO instance that has EXS' channel input, play something, and then return to the EXS.
Sometimes instead of losing the signal, the EXS delays playback. But that has never lasted for long. It's the beginning of the end.
I also tried outputing the entire EXS signal to a bus, then insterting the VE Pro Audio Input in the bus. No difference.
SCENARIO 2: EXS to VE PRO Instance with Nothing but a Channel Input
Identical, except instead of a loss of EXS audio, I get garbled audio, again after I hit play. And eventually even that is lost.
There are no loaded instruments in the VE PRO in this case. But just assigning the software instrument to an Arrange track and selecting it, then re-selecting the EXS track returns the audio.
This is curious, because if I just select and do not play the loaded VE PRO instance in the first scenario, that is not enough to coax back an EXS signal.