I think it was the perfect way to show that Aemond is so drunk for power and clout, not to mention being vengeful and hateful, that he'll make actions before he fully thinks them out. He thought he was just fucking with Lucerys. Lucerys, rightfully, thought Aemond was not going to stop until Aemond felt like he could lose something. Thus, after Lucerys attacks Aemond and Vhagar just loses his shit and Aemond is unable to control Vhagar. Remember, Aemond is not nearly as experienced as Lucerys, let alone any other dragon rider, and Vhagar isn't exactly the dragon with training wheels.