How do you make vocals sound like an instrument?

As the title says.

I would like to change some annoying vocals in a song to make them sound as like something produced by a synthesiser.

No, that’s not exactly what it says in the title. If you have the vocal part of a song all by itself, then it’s a snap with the Vocoder Effect to make a singing guitar, for example.

However, separating a single voice from a mixed song is difficult/impossible.