In Trigun Maximum vol. 8, Knives specifically uses the word “imprisoning” to refer to the way Plants are exploited by humans. However, in the same volume, we see that Knives is keeping Plants in these grate-covered cells in the floor:
There's a special kind of indignity about the optics of these floor cells. A literal and metaphorical space of the subaltern; the Plants are always below Knives. In a regular cell, at least the captive can stand up when they talk to you. In the tanks where Plants are ordinarily kept, they can even position themselves above Knives, as shown here:
There couldn't be more of a contrast to the visuals of volume 8, where Knives is even drawn walking over the dependent Plants’ cell.
And I don't think it's a coincidence that this panel comes immediately before the panel where Vash is revealed to be held captive in a cell that is identical except for being far smaller. (Neither Vash nor the dependent Plants have a lot of space. Vash just has the tiny cell to himself, while several dependent Plants are squashed together in an amorphous mass in the larger cell.)
I don't think Knives' hypocrisy could be any more clearly illustrated than it is through this imagery. There's no word for the way he's treating his brother and sisters other than "imprisonment".
And yet there's so much room for interpretation. Knives is explicitly shown herding the dependent Plants into their cell, but it's implied that Legato wrangled Vash into his (Knives leaves Vash to Legato whenever he's done talking to him).
Is this because Knives cares less about how Vash is treated? Leaving him in the hands of Legato, a human he knows to be sadistic, especially towards Vash. Or is it ironically optics again? If Legato is the one holding Vash captive, that's an Evil Human imprisoning Vash. The imagery neatly upholds the arguments Knives has been throwing at Vash, even if the context makes it all absurd.
Vash is not afraid to call Knives out, as we've seen in this volume. If Knives were to physically drag him off to a cell after all these sermons about humans imprisoning Plants, we can imagine it would be quite uncomfortable and embarrassing for Knives at best. Is Knives shutting down that debate before it happens? Refusing to have his worldview questioned anymore? Because his worldview is built on fear, delusions, extreme generalisations, and conveniently ignored nuance. Like Vash said, the Plants would not be exploited to this extent if Knives hadn't crashed the ships. And Knives doesn't want to hear it. He's operating on a cocktail of pure hypocrisy and sunk cost fallacy at this point. He simply lets Legato twist Vash into a pretzel for making that logic-based argument. The contradictions and flaws in his reasoning are starting to niggle at his mind, but he remains stubborn. This is illustrated perfectly by the scene where he hugs a dead Plant and imprints the image into his psyche, explicitly as a way to motivate himself (which implies that he's actually considering giving up ...)
All in all, I think this tells us that it's not lost on him how bad his actions are starting to look, and that it's really getting to him.