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I BELIEVE THIS IS VADER FEIGNING IGNORANCE OF LUKE'S EXISTENCE IN ORDER TO KEEP THE EMPEROR FROM SUSPECTING VADER'S PLANS.
Oh, come on. Thats just ridiculous! In the very next sentence Vader says he is just a boy, Obi-wan can no longer help him. Proving that Vader knew of him all along. The whole thing just makes no sence. Lucas fucked up! The Emperor is not stupid. Nor is Vader.