When he was still a dragon egg, Vermax was given to the sons of Rhaenyra, the would-be Targaryen heiress and daughter of Viserys I. Vermax hatches and bonds with Jacaerys, one of the sons of Rhaenyra and her first husband, Laenor Velaryon. During the Dance of the Dragons, Jacaerys rides Vermax into battle. Ultimately, both dragon and rider perish in an attack gone wrong, although accounts differ as to what exactly led to their deaths.
Vermax should show up at some point during "House of the Dragon," given his close ties to the main characters.