"When they had fought, he had also tried to strike the boy down, but that had been merely a test. Had he been able to kill Luke easily, Luke would not have been worth the effort to recruit. But although he certainly attempted to defeat Luke, the boy had held his own. Despite Vader’s superior skill, despite his experience, Luke had survived with no more damage than an easily repaired amputated hand.
The meeting had made Vader feel, not a normal occurrence lately. There had been the thrill at meeting a worthy opponent and pride that the one so strongly opposing him was his own son.”
“If Vader hadn’t broken that anger with fear and confusion by telling the boy that he was his father, Luke could have defeated him.”
Both from SotE pg 48.
And from the RotJ script.
"Luke and Vader are engaged in a man-to-man duel of lightsabers
even more vicious then the battle on Bespin. But the young Jedi
has grown stronger in the interim, and now the advantage shifts
to him. Vader is forced back, losing his balance, and is knocked
down the stairs. Luke stands at the top of the stairs, ready to
attack."
and
"Luke ignites his lightsaber and screams in anger, rushing at his
father with a frenzy we have not seen before. Sparks fly as Luke
and Vader fight in the cramped area. Luke's hatred forces Vader
to retreat out of the low area and across a bridge overlooking a
vast elevator shaft. Each stroke of Luke's sword drives his
father further toward defeat.
The Dark Lord is knocked to his knees, and as he raises his sword
to block another onslaught, Luke slashes Vader's right hand off
at the wrist, causing metal and electronic parts to fly from the
mechanical stump. Vader's sword clatters uselessly away, over the
edge of the platform and into the bottomless shaft below. Luke
moves over Vader and holds the blade of his sword to the Dark
Lord's throat. The Emperor watches with uncontrollable, pleased
agitation."
Here it seems Vader was defenseless against Luke's onslaught.