"Why are you here." It wasn't even a question, more like How dare you show your face to me.
It stung Riku a bit, coming from that face. "I want answers," he said.
"And I didn't give you enough?" For the first time since achieving Ultimate Form, Vanitas' feet touched the ground, though his blades remain pointed in Riku's direction. "You kill me, Sora dies. Sora is alive, somewhere. That's it."
On the other side of the coin, Qathkeeper quaked at the little word dies. Even back when survival was an either-or, Roxas couldn't kill Sora. Xion couldn't kill Sora. No one wanted to. Regardless, Roxas griped his Keyblade tighter. "I wasn't finished."
Xion held the blunt side of her blade in front of her friend. "You are now."
"I see," Vanitas grinned, "So that's your power of friendship. A numbers game." The blades scattered into sparks and Ultima returned to Vanitas hand. His clothes reverted to their original appearance, marking that the time for fun had run out. "How boring. None of you are giving it your all."
Roxas pushed Xion's Kingdom Key to the side. "You tried to kill us because you were bored?!"
Vanitas laughed when Xion grabbed Roxas by the hood instead. Riku did nothing. "What about it? I wasn't really aiming to kill." Why should he? He usually had reasons to kill people, and today he didn't. Simple. "The only thing I tried to kill was some time. Heartless are too weak. You were a bit better."
Extra emphasis on a bit.
Finally, Riku decided to participate in the conversation he started, which was typically expected of people. "You really don't know anything else?"
"Really!" He clenched his fists, and his heart thudded heavier. "You're not worth fighting, so go away!
There was one main reason Vanitas hesitated to fight Riku, outside of fighting three on one.
He'd never seen Sora fight Riku at his full potential, Keyblade and Master title included, and win.