And yeah, he definitely didn’t do Brackett any favors by withholding whatever his plans at that moment were, because I think the biggest weakness of Brackett’s draft as a story (so aside from not really getting the feel of Star Wars right) is that she’s clearly having trouble getting a handle on Darth Vader as a character, struggling to find a throughline for his various motivations. What she seems to settle on is that Vader is insecure - first he’s obsessed with killing Luke because Luke’s victory over the Death Star humiliated him; then he wants to to turn Luke to his side because he’s tired of being afraid of the Emperor.
This is how I interpret Vader’s motivations in Brackett’s draft: In Act I, Vader’s is just a straightforward villain carrying out instructions to kill Luke. But then in Act II, two things happen that change Vader’s motivation: (1) he talks with the Emperor, where we learn Vader is afraid of his boss and there’s tension between the two of them, and (2) he discovers Luke is becoming very powerful while training on Dagobah. These two things make Vader decide to try and convert Luke to the Dark Side, so the two of them can team up and overthrow the Emperor. Vader believes he has a good chance of converting Luke, because of course Vader killed Luke’s father in this version, and thus can leverage this fact to harness Luke’s rage.
There’s nothing really wrong with this - it’s a good a way as any to handle Vader’s character arc, given the information Brackett had about Vader while writing this. It’s certainly not as good as the final version of ESB, but it more or less works.
Another semi-related thing I find interesting: in Brackett’s draft, there’s no “cave scene” on Dagobah. Instead, Vader appears to Luke through some kind of “force vision” and the two of them have a conversation, where Vader begins trying to convert Luke to the Dark Side. It reminds me of the Sequels, where Kylo Ren speaks with Rey via the Force while she trains. It’s a convenient (if clumsy) way to have the hero and villain communicate before their big showdown later. Although, Brackett portrays the conversation between Luke and Vader as more dream-like, whereas in the Sequels it comes off as a straightforward Force Zoom call.