There's a very good reason why your stereo mic cable works. If the stereo mic is wired up such that the tip is one side and the ring is the other with the sleeve common, you'll get exactly what Yamaha planned - a mono mic directed to both channels which can give a stereo effect. They don't list the Vocal Harmony kit in the catalog but it's the same for all keyboards - a mono mic. And as I wrote in a previous post, they use a stereo jack so that the ring connector can be used as a switch to short the input to ground to prevent noise if no mic is plugged in and the input volume is turned up.

Bryan