Well, Ven already knew about Sora losing his heart - or at least he knew that a Nobody was formed when a person with a strong will lost his heart, and Roxas was Sora's Nobody, so he'd have to be a total idiot to not have put that together. But that wasn't the important thing at the moment, so there was no point in bringing it up.

"I don't know what happened to him in that last fight - I was going to confront him, but Terra stopped me and then all five of us were just all over the place. And I jumped to hit him, but he just grabbed me out of thin air and gripped my helmet hard enough to crack it and froze my body over completely, then tossed me off a cliff. Vanitas came down to finish me and Aqua, and then I had to face him..."

And then he died. Or lost his heart. Or came to this place. Whatever, the point was that Ven didn't know what the final outcome of the fight was, only that he'd taken Vanitas out of the picture before it could be finished.

"Riku said that you and he fought his Heartless and his Nobody, Sora - do you think you could draw a picture of him?"