The vapor cone is created by a Prandtl-Glauert singularity. It's a condensation cloud created when a layer of water droplets gets trapped between two high-pressure surfaces of air. In this case, the water droplets are caught between two crests of sound waves produced by the aircraft.
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Boeing F/A-18F
Super Hornet
This F/A-18F has produced a vapor cone, thanks to the right combination of speed, humidity, and air temperature. Despite what people say, it is not generated by the aircraft breaking the sound barrier.
Sorry the image isn't as sharp as it should be.