I thought Vader was lying through his ass. I didn't want him to be Luke's father. Of course, I loved the way it turned out in ROTJ. But back before ROTJ I had debates in school in which I vehemently championed the like-fuck-is-Vader-Luke's-father argument. Which was dumb, because it's clear by the way Luke and Vader relate through the force when Luke is escaping in the Falcon that Vader is the real deal. Luke can clearly tell with the force that the guy's his father. And if you think about it, his impassioned "Noooo!" in Bespin comes off like a frantic attempt to deny something he knows is true.