"Aqua... Ven... Keyblade... Who are you? I can feel it - we have met before. But when... No... it isn't you... It isn't you that I have chosen. Why isn't it him? Xe...ha...nort... Is that you? Xeha...nort... Xehanort!"
I think I know why he called Sora Xehanort. Since Vanitas returned to Ventus's heart and Ventus's Heart is inside Sora. It seems he can sense a little bit of Vanitas in him. Since Vanitas is the apprentice of MX they probably share some personality.
"I see. Your strength is... What I sensed in you is..."
I think what he's trying to say that he had Ventus's Heart in him. Since he does had Ventus's Heart.
I think I know why he called Sora Xehanort. Since Vanitas returned to Ventus's heart and Ventus's Heart is inside Sora. It seems he can sense a little bit of Vanitas in him. Since Vanitas is the apprentice of MX they probably share some personality.
I think what he's trying to say that he had Ventus's Heart in him. Since he does had Ventus's Heart.