I recently saw the double bill of Star Wars and The Empire Strikes Back
at a local huge screen 70 mm theatre from the second row. Wow!
Anyway, on with the story. As you all know (and if you don't you ignored
my spoiler warning) Vader claims that he is Luke's papa. Here are some
facts I picked up from the movies.
OB1: says Vader trained with him and was turned. Later Vader confirms this
when they fight.
Vader: has no concern that is "son" Luke is on tatooine, and he would know
this considering that the Force predicted he would kill the emporor.
Orders death of own "Brother" and wife without even thinking about it,
luke would likely have been caught for all he knew.
When firing at luke in battle over death star, he doesn't know who he is fighting,
just says that the
"Force is strong in this one".
The clincher is that Yoda states that he trained Obiwan and he also trained
Luke's father. But Vader was Obiwan's apprentice. For further evidence,
the emporor calls Luke the "Son of Skywalker" and he really would not call
him that in private conversation with Vader if Skywalker == Vader.
Finally, who is more likely to lie?