Let me clarify my reply. I feel as the song is now, there is no room for a vocal. Which is the premise of the post. If the single note piano and the harpsicordish sound parts are what is supposed to be a melody then laying a vocal on top would make the song more busy and they would collide with one another in the mix. Just my opinion. If you strip those parts away then a vocal would fit better. And the piano / harpsicord notes could be used to develop a vocal melody. But you need that space and can always add something that doesn't collide with the vocal to give it some more color or texture.
The piano chords, although redundant, are a good basis for the song. Could you mix in another progression as a bridge or chorus? Absolutely! Is it necessary? That's subjective to one's taste. A repeating pattern would need some window dressing musically or some high points vocally to keep it from getting boring. Again. my opinion.
On it's face it's not a bad piece of music. Some tweeking audio wise as Rob and others point out and panning would definitely help. Anyone one can slap sounds on top of one another. Giving those sounds their own space while making them cohesive is one of the many variables associated with the recording/mixing process.
But in the end it's your song. Keeping it an instrumental or add a vocal is your call, and what sounds good to you is all that matters.