The vr15 is smaller than the Vr12. The Vr15 features two motors, one in the vibrating teaser and one in the rotating shaft. The Vr12 features three motors, one in the vibrating shaft, vibrating teaser and rotating shaft.
The Vr15 is fully waterproof while the Vr12 is only splash proof.
The charging points are different as well. The Vr15 has a cord that is inserted into the actual item and the Vr12 has a charging base.
Hope this answers all your questions.