Vader is a weathly businessman under Palpatine. He's cold and ruthless. Until he meets Luke, who's like a ray of sunshine (Luke could be working part time somewhere). Somehow he feels relaxed when he's with him. Vader is intrigued and asks Luke out. Luke's kind of interested and agrees as he was always interested in older men. And they start dating. Then Vader becomes his sugar daddy as he spoils Luke with all sorts of gifts and even offers to pay his university tuition, etc. They both feel a mysterious connection with each other. Well into their relationship, Vader finds out that Luke is his son...

It's up to you if he reveals his relation to Luke or not.
They could even stay together, knowing they're related and do all sorts of kinky roleplaying together.

It would be great if Vader's obsession with Luke is explored.