They are hall effect sensors that detect the position of the rotor. It helps the ESC with the timing and "firing" of the phases to make the motor run smoothly. It helps most at low speeds and when starting from a dead stop, where some sensorless motors will "cog" and stutter because they don't have the sensors to tell the ESC rotor position.

Sensorless operation is actually more efficient than sensored, so many newer ESCs use a combination mode where it starts the motor using the sensor, then switches to sensorless at higher speeds for better efficiency. Not sure if the VXL is capable of that though.