So, I was wondering. Why is that in the opening of Birth By Sleep, it shows Ven holding an orb of light and then dropping it? I sometimes told myself that it was his heart or something but it didnt make sense to me. What exactly does it mean or symbolize?

fool. probably his heart. There is a lot of symbolism in the openings.

his heart, because near the end, ven realizes that if he destroys vanitas, then that destroys his own heart, so it shows his decision by choosing to give up his own heart to destroy the X-blade and Vanitas. but instead, Ven got lucky and because of his connection with sora, he didnt disappear, but just went into sora's heart.

It symbolizes Ventus's heart finding it's way to harbor with another heart (Sora's heart) in order for Ven's to recover. Shortly after Ven and Vanitas' face-off in their combinined heart.

what they said