Thanks, Corky.
I should have explicitly said that the VB3fx audio output is routed to a physical audio output.
There isn’t anything wrong with my Output A port - it’s available and working, as I can tell if I re-route audio output from VB3 (first plugin in the chain) direct to Output A. In any case, all works as I want before I save and re-load the song.
In the Organ Plugins thread several people have used the VB3fx Leslie as a Leslie effect for other organ plugins. But I don’t think any have used VB3 as the “source” organ plugin. So I suspect my problem may be to do with an issue I know @Brad has had to workaround before, which is that both VB3 and VB3fx report the same plugin id… or may be another VB3 “quirk”.