I have found the following setup useful. Maybe you will too. I welcome any suggestions for improvement.
Let's say you want to create a session for a string quartet, consisting of 2 violins, 1 viola, and 1 cello.
1. In Pro Tools (PT), create 4 stereo instrument tracks.
2. Instantiate Vienna Ensemble (VE) on the first instrument track.
3. In PT, assign the MIDI OUT (not AUDIO OUT) of tracks 1-4 to VE MIDI channels 1-4.
4. In VE, create 4 channel strips and name them (Violin 1, Violin 2, etc.).
5. In VE, assign the MIDI IN of each channel strip to MIDI channels 1-4. (This connects the MIDI OUT from PT to VE.)
6. In VE, load your samples into each channel (violin, viola, cello, etc.).
7. In VE, assign the AUDIO OUT of the Channels as follows (This is found at the bottom of each channel, in a small drop-down menu.): Channel 1 to 1/2, Channel 2 to 3/4, Channel 3 to 5/6, Channel 4 to 7/8, etc. (This sends the audio of VE out.)
8. In PT, assign the AUDIO IN (not MIDI IN) of each track as follows: track 2 from "VE-Stereo 2," track 3 from "VE-Stereo 3," track 4 from "VE-Stereo 4," etc. By default, track 1 receives audio from "VE-Stereo 1." (This connects the audio from VE to PT.)
9. You should now be able to select each track in PT, and play the samples in VE using a MIDI controller.
10. At this point, you may want to also create a stereo aux track, instantiate a reverb unit on it, and create sends from each track to the aux track. This will give some reverb to the samples, and you can control the amount using the send on each track.
This setup allows you to mute, solo, and mix in PT rather than VE. You could still mix in VE if you want to, but since the PT transport doesn't work when in VE, mixing in PT is easier to me.
Regards,
Gary