If I recall correctly Vader didn't have the chance to kill him in ESB.
Plenty of times.
Right off the bat, one hit sends Luke flying down the Stairs.
When Vader edged Luke into the Carbon freezing. One thrust would have killed the saberless Luke.
When Vader points the saber to Luke's throat and says "You are beaten". One thrust = dead Luke.
When Vader sawed off his hand. One hack after it and GG Luke
When Vader was owning him with the objects.
Vader was at first trying to turn Luke but he jumped. What was Vader suppose to do? Jump down after him?
No, cut him up before he gets there.
Fact is Luke won fair and square. Vader was scared. You could even tell from the way his breathing changed when Luke charged him.
No... He just didn't want to fight his son.