Both heading and angular velocity, which gyroscope is measuring, are identical at any point of the rigid body that rotates and translates in plane. In theory, MEMS gyroscopes may be affected by the accelerations that are higher further away from the center of robot rotation.
However, since your robot doesn’t experience crazy high accelerations, you should be fine mounting vex gyro at any location on your robot.
Also, if your goal is to achieve repeatable autonomous movement and not to measure absolute precise value of angular velocity at every moment of time, the exact placement doesn’t really matter.
What matters more is isolating the gyro from the sources of mechanical vibrations: