That doesn't say that Vanitas was faceless....it only says that he would've looked like whoever connected their heart with Ventus.
Obviously the model used for when Vanitas was "born" was him in his mask, since it would've meant another model would've had to be put in, and they also still used HJO for "just born Vanitas", which is strange actually since Ven and Vanitas had no connection to Sora at this time, so he should've had a different voice than Sora's really. But again, that would've meant they'd have needed another VA, it would've been too much, and would've confused everybody by giving him a different voice for that one scene and never have him talking like that again.
But anyways, what you said doesn't say that Vanitas was faceless before Sora connected with Ven, just says that Vanitas could've looked just like Riku, had his heart had connected with Ven's. But since Riku was born roughly a year before Sora, that would be challenging, since the only reason that Sora and Ven connected was due to it being the moment Sora was born (Actually when his heart developed, but let's not get into that).
So like I said, in theory Vanitas should've looked exactly like Ven when he was born, before Sora connected with them. Since Vanitas took the dark half of Ven's heart, and his hair color is just to show that he was the dark half. And the heart affects the appearance, and Ven's heart makes him look the way he does, and makes Roxas look the way he did, so since Vanitas and Ven's hearts were the same at this time, he should've looked exactly like Ven only with black hair.
One example would be that everyone forgot about him throughout the one year that we was asleep.
Cause not only did Namine rearrange his memories, but because of that it also affected everyone else's memories of Sora, Donald, and Goofy, since she had to reconstruct their hearts/memories really.
