I don��t think Aemond ever truly controlled Vhagar.
People act like Storm’s End was the first time he realized it.
Vhagar only accepted him because she was grieving Laena. It was a bond born out of circumstance, not the deep connection we see with riders like Daenerys or even Caraxes and Daemon.
And Vhagar never really let him forget that.
Even before Storm’s End, she constantly felt difficult to handle. At Rook’s Rest she was impatient, during the chase after Silverwing she ignored what Aemond wanted more than once, and every time she was in the air it never felt like Aemond was commanding the largest dragon in the world but more like he was desperately trying to persuade her.
Storm’s End wasn’t the moment Aemond lost control, that was the moment he realized he never truly had the control he thought he had
Maybe the real rider of Vhagar was always Vhagar herself.