Father Skywalker said:
Washington Post in 1980:
So, darth vader had an elaborate scheme and plan involving capturing/kidnapping Han Solo and Princess Leia by making a deal with Lando Calirissian at Cloud City, so that Luke would go and rescue them, and he could confront Luke and tempt the young Skywalker to the Darkside of the force, to become a dark lord of the sith.
I always read it as Vader thinking there are now only two master Force users left (now that Ben is dead) and that Luke is his only hope of getting rid of the other one.
He doesn't plan on capturing Leia, Han, Chewie and Threepio.
He assumes Luke is onboard (he associates the ship with Luke not Leia) because he can sense 'him'.
When he finds only his friends on the Falcon (a ship he is very familiar with remember) he sets about torturing the Falcon crew because he knows Luke will be able to feel their pain.
When the PT came out this became replaced by the rule of two (or rule of as many as Dooku wants in the EU).