MIDI protocol, including both hardware and software, is a key connecting element in triggering VSTs.
Typical note mapping:
Windows is reported to have an issue with a number of registered MIDI devices. This is limited to 10 and results in a funny situation. Once you have filled the slots, the new devices may not be recognized. This is particularly annoying when simply switching the same device to a different USB port.
Unused MIDI devices need to be removed to free up space for new ones.
The simplest solution for this is to open Device Manager, expand Audio Devices, and turn on Show Hidden Devices from the menu. Then simply "uninstall" the unused devices.