Is it possible to use the HarmonyControl Guitar to control VST synths / virtual instruments, if I connect its MIDI output to a MIDI input of a MIDI interface?
Technically, you can use the HarmonyControl Guitar to do the "guitar to MIDI" thing with a VST synth or virtual instrument.
However, the main difference from other guitar-to-midi devices is that HarmonyControl Guitar never outputs verbatim what the guitar is playing. It outputs chords only and will not re-send a chord that has already been played.
As such, the HarmonyControl Guitar is very suitable for use with a TC Helicon harmonizer, but less suitable for use with virtual instruments.