Seriously? Over-analyzing - The retard side of theories.
Ven, is short of Ventus, not because it is significant, but because it's simply a nickname. Another reason for them call him Ven is because Ventus and Vanitas sound and look alike. They probably wanted something to seperate them, so they gave a nickname to one of them.
If you theory was even remotely true, Nomura would have mentioned something like "The reason why his name is shortened is somewhat important" instead of just "Ven is a nickname that is short for Ventus".
Ven, is short of Ventus, not because it is significant, but because it's simply a nickname. Another reason for them call him Ven is because Ventus and Vanitas sound and look alike. They probably wanted something to seperate them, so they gave a nickname to one of them.
If you theory was even remotely true, Nomura would have mentioned something like "The reason why his name is shortened is somewhat important" instead of just "Ven is a nickname that is short for Ventus".