It's already been proven that the Vortex transmits MIDI and your MODX receives MIDI from it. Because the Vortex was sounding one Part earlier. So all of the underpinnings are there.
What does it mean that you set the MIDI channel on "each of the parts"? To me, this sounds like you changed some setting in each Part available. So if you had 4 Parts on MODX - then you went into each of the 4 Parts and changed something in Part 1 to match a MIDI channel, something in Part 2 to match a MIDI channel, Part 3, Part 4. 4 different settings. And if this is what you did, then you were changing the MIDI channel of the wrong thing - Transmit on MODX and not Receive.
What you need to do is set one global parameter for the MIDI channel that applies to all Parts. And you also, if you've changed this, need to undo the zone control settings by turning zone control off. I would also turn off Part-level zone control. This is because zone control defeats single-channel mode. The settings "Single" or "Hybrid" MIDI channel mode are both mutually exclusive with Zone Control - and Zone Control always takes precedence if these are both set (turning off Single or Hybrid).
This shows what should change. Maybe this is already happening and you were not using zone control to set the MIDI channels. But if you didn't use the above menu - then you need to change what's shown as "Hybrid" to "Single" and the red box change to your MIDI channel (the one that matches the Vortex's transmit channel). Notice how there is a red arrow from this red MIDI channel setting box that leads to all of the Parts with Keyboard Control = On. The Parts with Kbd Ctrl = ON will be assigned to this single (same) MIDI channel ("MIDI I/O Ch" ) and there is no need to make the change in any more than this one place.
Again, be sure that zone control is turned off to avoid defeating single channel mode.