Or, in V4 you can switch track's sliders to send MIDI CC7/CC10.
One instance on each MIDI track leading to the instrument, set to the appropriate channel.
If you want to set a MIDI volume for each individual channel of a multichannel VSTi, best use ReaControlMIDI. I can mute/solo all tracks which are routed as outputs of a multiout VSTi with no problem.
