The Sliders on the MODX are dedicated to their roles as
1) Part Volumes for PARTs 1-16 (in four groups of four)
2) Individual Oscillator Levels, Elements 1-8 or Operators 1-8, (toggle 1-4/5-8)

_They function as Part Volume when “Performance Control” is selected. (When acting as Part Volume, they can send cc#7
_They function Element/Operator Level when “Part Control” is selected.

There are 8 Assignable Knobs per MODX Part slot, these can be freely assigned to transmit any cc numbers between 1 and 95... on a per Performance basis.
With the MODX, it would be wise to use the “Zone Master” function to setup one of the Part slots as an External Zone. The Zone Master function allows you to configure per controlled device, exactly what gets sent Out. You can control as many as eight Zones (a Zone can be an internal Part or external device).

An external Part setup is done by placing a ‘dummy’ MODX instrument in the Performance Part slot, then deactivating it by setting “Int Sw” = Off. The Part slot can then be used exclusively to control an external device (like your VSTi), on whatever channel you select. You can use the “Zone Settings” and “Zone Transmit” screens to customize what goes Out via MIDI to the the VSTi.

Extra Credit: FYI
The MONTAGE features a DAW REMOTE function, not found in the MODX, that allows the front panel Sliders and Knobs to be used to control Plug-Ins within the DAW environment. (Cubase, Logic, Ableton, Pro Tools)

Added in MONTAGE firmware 2.50 — Using the new DAW REMOTE Feature