There's something about Darius that just makes Vladimir roll his eyes every time he gets to that point of the conversation with him.
He doesn't dislike him. He doesn't even hate him. Hatred within Vladimir is reserved for those who burn him and fill his thoughts with violent, violent things. For Darius, it's possibly in the way he looks - in his eyes, his teeth, his posture, that catches Vladimir's temper like a hook through the lip.
He has hideous opinions when they're around the war table and there's an empire's sovereign mediating their discussions. But he's not terrible company. Could always be worse, could always be the younger one. But he's still -
Doing that thing where he pulls his chest up and straightens his back and acts like he has so much height on Vladimir. He has his arms crossed over his chest, and no matter how Vladimir frowns at him, narrows his eyes and fixates a glare, he looks like he's proud of himself. Whatever conversational point was being made has left them. Vladimir mirrors his pose.
However, he makes the mistake of leaning himself forward, just enough that Vladimir notes the fabric of his collar hangs down his throat. Whatever he's trying to say, maybe a what's your point now, or a have nothing to say?, because Darius is a fucking child sometimes - it doesn't register in him.
It's not the bickering that heightens his interest, and what he does stays off record - but there's one thing Vladimir's a little into.
It's found in the way he reaches up, with all claws and silver and intent, and yanks him down those few inches that keeps Darius from looking Vladimir in the eye. Vladimir closes the step-sized gap between him and finds the mouth he wants, rolling his mouth into his with enough of a snarl to catch Darius' lower lip. He holds him there, pulling him forward, harsh enough that Darius' stunned form has to take a step forward. When he pulls himself away, he pulls on his lip hard enough between his teeth to mark him with a warning.
It's not swollen, but reddens, swells underneath with agitated blood. Darius looks so stupid when he's stunned like this.
Vladimir's smirk says, keep that in mind.