Interesting discussion, and I've always looked at it as Darth Vader not really wanting to kill Luke, but rather have him join him and turn against the Emperor. He was taunting him in order to turn him towards the dark side, but when it came down to a powerful jedi (at this point, Luke has been training for nearly 10 years according to a timeline from GL) who tapped into his anger, Darth Vader realized the only way he could stop Luke was to kill him, and with that conflict already present in his mind, he couldn't do it.