Summary
- Darth Vader inadvertently sets up the possibility of his redemption by suggesting to Palpatine that they turn Luke to the dark side instead of destroying him, going against Sith norms and risking his own position.
- Vader's decision to bring Luke into the fold as a Sith creates a power struggle between the Skywalkers and Palpatine, in violation of the Rule of Two.
- Vader's unwillingness to harm Luke in The Empire Strikes Back shows that he values his son's life and potential for good more than his loyalty to the Sith, paving the way for his eventual redemption.
Darth Vader shockingly redeemed himself in Return of the Jedi, but it was The Empire Strikes Back that set up the possibility by defying Sith norms. As Yoda said, "always two there are; a master and an apprentice." Palpatine expanded on this principle of the Rule of Two, proclaiming that the master possess the power and the student is the one to crave it. The relationship between Vader and Palpatine embodied all of that and more, with both knowing the former could never truly usurp his master given the physical limitations that hindered him.
Despite that, Vader was willing to make power moves on a number of occasions. Largely to the amusement of Palpatine, his apprentice amassed an underworld army after the events of A New Hope with designs of destroying his master. He took similar action after learning the truth of Padmé's death before Return of the Jedi, though Palpatine easily defeated Vader and demonstrated his true power. While Vader never actively sought a path to redemption, he actually opened his mind to the possibility in The Empire Strikes Back, albeit through the pursuit of ruling the galaxy.
Your browser does not support the video tag. Darth Vader Didn't Want Luke Skywalker To Be Destroyed 
When Vader communicates with Palpatine in The Empire Strikes Back, the Emperor is quick to let it be known that he fears Luke's power and potential to destroy the Sith. Vader sees where the discussion is headed and steers it away, first trying to spin Luke as helpless without his masters before suggesting that they pursue turning him to the dark side. The Emperor, undoubtedly having considered this already, agrees with Vader's plan, though both Sith are aware that they're making a power play.
With the additional implication that Anakin Skywalker is no more, Palpatine believes Vader is his. However, the piece of Anakin that remains in Vader doesn't want to see his son get killed. Luke is a fresh start for Vader, a beacon of hope that doesn't come with the terrible memories of his past. Though he's initially seeking to gain Luke as an ally for the wrong reasons, Vader is unwittingly paving the road for his own redemption. He can't bring himself to destroy Luke at Cloud City, despite claiming that his son will die if he refuses to join the Sith, and Vader would rather violate Sith tradition than see that happen.
