This is just going from the films, with a heavy emphasis on the original trilogy:
When Yoda says the Force isn't about being a great warrior, he isn't just spouting some hippie nonsense. Vader understands this, too, but from a darker perspective: He has Luke's friends' lives to bargain with, or at least he thought he did. More than that, though, at any point Vader can win the fight just by telling Luke the truth: he is Luke's father.
Vader and the Emperor both see Luke as a valuable asset, to be preserved and exploited if at all possible. Vader, however, has a strong idea that once the Emperor has Luke, he'll no longer need Vader, at which point Vader himself goes from being an asset to a liability. Not a safe position to be in when your boss likes to microwave people with his hands.
So I don't think Vader was ever intending to directly hand Luke over to the Emperor. Vader's primary goal wasn't so much to capture Luke as to keep him for himself, or at least out of the hands of the Emperor, which is, I think, one of the reasons that Vader lets him escape. If nothing else, he's preserved the balance of power.
Because the only being in the galaxy Vader is truly afraid of is the Emperor, which is why Vader doesn't just kill the sick SOB and take control of things himself. Vader, after all, isn't truly evil, he's just a guy who wants to bring order to the galaxy.
And he knows the Rebel Alliance isn't a match for a psychotic near-diety who can shoot friggin' lighting out of his hands; their assault against the first Death Star would have been a failure if it hadn't been for this one kid whom the Force seemed to exceptionally favor.
A kid who it turns out has the power to destroy the Emperor, which the Emperor himself has foreseen. And best part of it is, he's Vader's son! If they team up together they can kill the Emperor, end the senseless war, and rule the galaxy as father and son.
It's a seductive proposition, and Vader was hoping it would have the added force of having Luke and his friends securely in his custody.