First his mom changed his name:

his mother changed his name to James David Hamel to remove his father's first name and surname and to preserve an uncle's first name, David.

But his mother was a drug addict so he was raised by his grandparents:

He and his elder sister, Lindsey, were raised primarily by their maternal grandparents, James and Bonnie Vance (née Blanton), whom they called "Papaw" and "Mamaw"

He changed his name before graduation.

Although Hillbilly Elegy states that he adopted his grandparents' surname of Vance upon his marriage in 2014,[8][27] the name change actually occurred in April 2013, as he was about to graduate from Yale

Quite a joke that he changed his name from an abusive prick called Donald, only to cuck for another abusive bastard called Donald later in life.