I know a lot of people got attached to a lot of ideas about how the Vader scene would go and are wondering what the point of it was if he didn’t even speak, and so my answer to that is: because it’s not about him.

That moment is not about Anakin Skywalker, his feelings about Barriss Offee, or Barriss Offee’s feelings about him. Anakin-as-Vader does not give a shit about her, which is to say he hates her the same way he hates everyone he meets. He puts the blame for Ahsoka leaving him squarely at Ahsoka’s feet, and hates her for it, wants her dead for it. Barriss doesn’t matter to him any more than any other Jedi he knew before the purge does. (If there’s any tinge of personal feeling, it’s more adjacent to satisfaction at seeing her stuck in the same despair trap as him than anything else.) And Barriss doesn’t have a clue who he is.

So: The point of that moment is that Barriss sees that she’s kneeling to a Sith Lord. She sees that things are so much worse than she had ever feared. She sees that she’s completely lost control of her life.

And in that moment, she cannot see the way out. She feels small and afraid and everyone she loves is dead and the entire oppressive structure of the Empire is bearing down on her and saying: If you want to live, this is what you have to be. This is all you can ever be. If you step a foot out of line you’ll be dead and it won’t even do anything to help anyone.

Later, when she gets back out in the galaxy and sees again the effect she can have on people, she’ll rekindle hope within her and see the way out. There was simply NO universe where Barriss “friend to all children” Offee could ever have been an Inquisitor past the moment where she actually had to fulfill the duties of one, and the whole point of this failed attempt to force her into the Inquisitorius was to show that was true; whether she ended that first mission dead or as another rogue Jedi on the run, she never would have been what the Grand Inquisitor wanted her to be.

But right then? Now? Kneeling to Vader, knowing that she’d be dead before she could even draw the new red lightsaber on her back—the one that she hates and that hates her back because she was forced to profane it herself?* When the Grand Inquisitor has kept her isolated and provoked her over and over again so that when she finally lashes out to defend herself he can tell her that’s who she really is? There’s nothing she can do. There’s nothing she can do.

That’s her lowest point. That’s the only moment she ever really was the First Sister.

(*Kyber crystal bleeding is the kind of thing that only really hits if you're a huge turbonerd about the star war lore and know how it works, so I get why they skipped over it onscreen when it's really just a more abstract metaphor for what she was forced to do in the pit fight, but given that we know Iskat and Reva both made theirs themselves there's no way they didn't make Barriss do it too.)