The problem: While sending one instance of Vienna Instrument Player through Vienna Ensemble to my DAW (Cubase 7.5), I was only able to hear the results when in the Cubase window itself – not while within the VE/VI window.
The solution: In Cubase (7.5), go to Device Setup/VST Audio System, and Uncheck “release driver when application is in background" (see attached image).
This seems counter-intuitive, but it is the solution.
This solution was corroborated by Andreas Olszewski:
“Please keep the following Cubase option turned off:
Devices/Sevice Setup/Release Drive when Application is in Background
"This way Cubase will keep the audio driver when you switch Windows and you will always be able to hear the loaded instruments, regardless which window is in the foreground.”