The Mod Wheel is freely definable and as such as no "sound" that you could hear in OpenMPT. Granted, the most common use case is probably vibrato but there are tons of other ways to use the mod wheel in MIDI-capable synthesizers (e.g. to change the filter cutoff). Tracker vibrato semantics are also very different from how vibrato would work with a mod wheel so it's not possible to directly translate those commands to mod wheel commands. If you want to have identical playback between the tracker and MIDI export, you will have to work with an instrument plugin to begin with, e.g. a soundfont player like sforzando. Then you can have MIDI semantics in the tracker in the same way as they would be used in the exported MIDI file.