So what you're trying to say here is:
A heart pure with darkness doesn't have a conscience.
I don't think that's true necessarily. Master Xehanort left Ven on the Islands out of compassion, even if he had Dark plans for the boy. That proves HE has a conscience,....a sense of right and wrong.
I believe that Light and Dark, are, like MX says, two sides of the same coin. What might be right for someone Dark would be wrong for someone alligned to the Light and vice-versa. Someone neutral for example, would take both sides into account and decide based on that criteria. Though we know Vanitas can think for himself, it isn't about that.
In Vanitas's eyes, forging the X-Blade is indeed the right thing to do. Whether he wants to usher in another Keyblade War, or destroy everything, that is what he thinks is right.
Therefore, if he were to have an enemy in his story, it would be the TAV team, since they are working for the Light, which in his view, is wrong. If he had a pre-BBS story, that enemy would be Master Xehanort, since his Dark Heart would still be developing and he would still be choosing between fighting for Light and Dark. Because I seriously doubt he's a total sadistic fool from the get-go, but more open and lifeless, like when Roxas first joined the Organization. Even if he's a being of pure Darkness, having a conscience gives him power over his life. (Of course, if he were to disobey Xehanort, he'd probably get killed or punished in some horrible way, so maybe that's an overstatement.) Vanitas's personality has to be conditioned somehow, hence training with Xehanort.