Since the game already came out in Japan, I think it is okay to believe a Japanese with a screenshot of the Japanese credits, as well. Vanitas does have black hair, in the same style as Sora's, his face looks like Sora's, but he has a mean expression always on his face, with his eyebrows in a frown-like position. His eyes are, correctly, orange/yellow.
I'm sorry, I should have researched more before writing my first reply.
The Vanitas and Ventus (real?) story:
Vanitas is a sort of Ventus "clone" created by Master Xehanorth. He is, in fact, the Ddarkness inside Ventus.
Somehow, Ventus was fused with Sora, so when Sora, in the first Kingdom Hearts game, used a keyblade on himself, Vens heart followed along to Roxas body. This would explain why Roxas looks like Ven so much, and also how he is able to feel emotions. So, actually, Roxas has Ven's heart.
