Yet another realm where I see things dimly. But it's possible that all CC#7's are grabbed as overall volume and not sent through, no matter what channel.
Try this. Create a fader object. Assign Output: Control. Channel (whichever channel you need). -1- 3 (that is, it sends the first MIDI byte as a 3). The fader's input is not relevant for this exercise.
Connect the fader to the VE Pro plug-in channel strip. To verify that your channel is being read within the VE Pro plug-in, move the fader and confirm the little green MIDI activity light on the far left of the instrument within VE Pro.
Now right-click on the volume slider of that channel's pane (far lower right of the big blue window with the big eye). It will blink red, waiting to be assigned. Move the fader again. The volume slider should see it and stop blinking.
So now a specific instrument within your plug-in will adjust volume to CC#3 of its assigned channel. Why not CC#7? I don't know. The volume slider doesn't accept it as assigned. In fact, it may not accept any of the dedicated CC's, from what I can see. Perhaps this is hard-wired, to avoid confusion.
Work backwards from here. Figure out some automation class that can drive SOMEthing that can be assigned per channel to the volume sliders. From my poking about, it seems that Fader and Meta classes cannot be assigned to an instrument's volume slider, only a CC not dedicated already.