It's possible Vader did save Luke.
When Luke lets go, he doesn't fall straight down:
INT. REACTOR SHAFT
Suddenly Luke is sucked into an exhaust pipe in the side of the shaft.
When Vader sees this, he turns and hurries off the platform.
There doesn't seem to be much air movement in the side vent, certainly not the gale-force wind which would be needed to pull Luke across the width of the main shaft.
There are at least three possibilities:
Vader actively saved Luke, by using the Force to push him into the side shaft;
Vader knew through Jedi precognition that Luke would survive his fall -- the opposite of "having a bad feeling about this";
Vader didn't know if Luke would survive, but regarded the fall as a test of Luke's Jedi skills -- maybe Luke used the Force to push himself into the side shaft.
After Luke is rescued by the Millennium Falcon, Vader wants to capture him:
INT. VADER'S STAR DESTROYER - BRIDGE
Vader stands on the bridge looking out the window as Admiral Piett
They'll be in range of our
tractor beam in moments, my lord.
Did your men deactivate the
hyperdrive on the Millennium
Yes, my lord.
Good. Prepare the boarding party
and set your weapons for stun.
But he is overconfident in his prospects of catching Luke, and didn't expect the hyperdrive on the Millennium Falcon to have been repaired, so Luke gets away.