I think the message was more Vanitas accepted at his core that he IS darkness and nothing else. Therefore, he wont go against what he believes the nature of Darkness is. Being Ven and Sora's darkness is his weird way of bonding with them...it's sad that he believed that to be his only path.

Hence, Ventus not wanting too, but let him choose his path, while Sora was trying to talk to him through the end cuz that's how Sora is. I hope they go into it one day in the future. I was hoping more for him too. Neat little Story here.