Hi katapulp,
...Am I missing something obvious?
Yes, I think the GND of the Arduino and the GND of the VMUSIC must be connected.
That is very obvious, but since I don't have a VMUSIC myself I can't tell about the rest of the wiring.
BTW any reason why you don't power the VMUSIC from the Arduino-USB-Supply instead of the external Power-Supply ?
Eberhard