A year ago, Sora and I were trapped for a while when Xehanort took over Terra's body, so we couldn't interfere.

What happened?

[Vanitas could potentially know the broad strokes, sure. He could know what sort of spell Xehanort had prepared. But he couldn't know that they argued, or that Roxas had nearly despaired, or how they'd found their way out. Only Sora would.]