Vin sank like a stone, he gave up control waiting for Ryan to command his body. Ryan did it without a word. He was fighting the fading mind control that Vin had exercised on him. He used his athletic prowess to exhaust Vin's body and to block Vin from reading his mind and the plan for revenge that Ryan had made.
He had scanned Vinnie's unguarded mind while he was Property of Vin Cost. Thanks to Sal's bleach job, he no longer was anyone's property. He knew how Vin had changed him into a jockstrap. He knew how he could change himself back, but he'd done more with Vin's pathetic life in the last three weeks than he'd done with his own in 18 years. He didn't want to give that up to go back to an alcoholic pair of parents, and a dead end job. Vin was up for a scholarship, and he had a family who really cared about him, and he also was an adept natural wizard. No, Ryan planned to take over Vin permanently. He began to think the spell softly in his mind, once he realized that Vin's consciousness had decided to take a nap.
When he emerged from the pool, Vin's body belonged to Ryan's mind, and Vinnie's mind was trapped in Ryan's transformed body as a jockstrap. At his locker, Vin hesitated for minutes agonizing as to whether the spell had worked, but he had nothing to lose. Vin needed him to stay on the water polo team, and keep his body honed. Ryan wasn't the dullest tool in the shed, when Vinnie had taken control during classes, Ryan had paid close attention. He had learned how to study from Vinnie, and that would pay off when he got to college. He stepped out of the jockstrap.
Vin sighed in relief. He was still in control. Vinnie was now the wet jockstrap. Ryan enjoyed the first long shower he had in days. When he was done dressing. He tossed his old jockstrap into the lost and found bin. He whispered, "Good bye, Vinnie. Hello, Vin's new life."