Okay, I have a theory about Ven and the Title Birth by Sleep. Here goes...
Ven is slowly losing his Heart. In one of the Trailers, he askes someone to "Erase him". But, instead of erasing him, they take his heart and place it in the body of the boy he chose to wield his Keyblade (Way to the Dawn) and "Goes to sleep, and becomes reborn. So, when Sora becomes a heartless, Vens Body doesn't have a container, and becomes Roxas. This is why he has emotions without a heart, because he is a soul.
Ven is slowly losing his Heart. In one of the Trailers, he askes someone to "Erase him". But, instead of erasing him, they take his heart and place it in the body of the boy he chose to wield his Keyblade (Way to the Dawn) and "Goes to sleep, and becomes reborn. So, when Sora becomes a heartless, Vens Body doesn't have a container, and becomes Roxas. This is why he has emotions without a heart, because he is a soul.