In the series, Daemon is the one who summons Vermithor and sings to him in Valerian in hopes that he will help out in the efforts against the greens, aka the faction who supports the ascent of Aegon II as King of the Seven Kingdoms. However, in Fire and Blood, Lord Gormon Massey is the first to approach — things don’t go well, and Vermithor burns him to death.
Source: https://www.distractify.com/p/who-rides-vermithor-house-of-the-dragon