Q: Is the VD-Mic stereo or mono?

The VD-Mic is mono.

Q: Does the Geniusmic draw power through the phone connector, or just for audio input?

It is just the connector for audio input into the phone.

Q: Will the Geniusmic be able to record sound when I am using DJI osmo 3 ?

The Geniusmic can not work with DJI Osmo 3.