De acordo com a nova (e canônica) romancização do filme, Luke trabalhou por si mesmo do contexto. Ele só sabe de um guerreiro Sith matador de Jedi e simplesmente assume que é o Vader que ele acabou de ver. Note que isso acontece antes ele fala com Leia.
“I CAN’T BELIEVE HE’S GONE.”
Those were the only words running through Luke’s head, caught in some horrible loop, as the ship lurched into space. I can’t believe he’s gone. He slumped onto the bench by the game board on the Millennium Falcon, his legs finally giving out under him. Luke couldn’t bring himself to move. He couldn’t close his eyes, either, not without seeing the way Ben had looked just before…
He’s dead, Luke thought. Why can’t I say it?
Why did he have to keep seeing the way Ben had deactivated his lightsaber and hadn’t tried to stop him—Darth Vader. The name hissed through Luke’s mind like smoke, making the hairs on the backs of his arms rise. Just seeing the man…the thing had been enough to make Luke feel as if he were drenched in ice. The shock had left Luke useless as a droid with its circuits fried.
Star Wars: A New Hope: The Princess, the Scoundrel, and the Farm Boy