BmB said:
I think you are ascribing too much compassion to the dark lord of the sith here.
Considering the last they saw of each other I'm sure he would have tried to take revenge, then scoop up the boy and train him for the dark side to help him overthrow the Emperor. Had he known.
Not so much compassion as obsession.
I know that what I'm suggesting isn’t what Lucas intended and isn’t consistent with the EU, but it would make sense to me.
What drove Anakin to the Dark Side in the first place was his obsession to protect Padmé. When Padmé dies giving birth to his son, I can easily see that obsession being transferred to Luke. So he becomes obsessed with protecting Luke.
Logically, there are four possible outcomes for Luke:
(1) Luke learns the Force, uses it for good, becomes a Jedi and fights against the Empire.
(2) Luke learns the Force, uses it for evil, succumbs to the Dark Side, and becomes a creature like his father who knows nothing but rage.
(3) Luke doesn't learn the Force, and lives the life of an ordinary man.
(4) Luke dies.
Vader wants to protect Luke the way he wanted to protect Padmé, so (3) is by far the preferred outcome. He allows Luke to live with the Larses, and he allows Obi-Wan to watch over him (but not to get too close to him). Obi-Wan also wants Luke to be OK, so he watches over Luke from a distance as a “crazy old hermit,” but doesn’t befriend Luke lest Luke learn about and become too interested in the Force, and thereby invite the attentions of Vader and Palpatine.
Then the events of ANH occur. Obi-Wan decides to teach Luke about the Force. He knows that by doing so he is putting Luke in great danger, but does it because the galaxy needs him.
Outcome (3) having been eliminated, outcomes (1), (2), and (4) remain as possibilities. Vader, because of his loyalty to the Emperor / Empire / Sith / Dark Side / self, rules out (1), leaving only (2) and (4) as acceptable outcomes, and (2) preferred to (4). This is when Vader determines that he must turn Luke or, failing to turn him, destroy him.