Vocal instrument as in ...
... it's MIDI. There's no human voice instrument in MIDI. And I'm not going to record myself singing over it.
How is that hard to understand?
And most people would listen to a MIDI and imagine what it could be, rather than what it is right now.
I mean, like you said, it's a MIDI.
And piano wouldn't work. It doesn't have the sustain anywhere near that of the human voice.