Interestingly enough, the name 'roll vault' is actually an oxymoron.
Vault: "To jump or leap over, especially with the aid of a support such as the hands or a pole."
Roll: "To move forward along a surface by revolving on an axis or by repeatedly turning over."
So, rolling is maintaining contact with the surface, whereas vaulting is going over the surface (except perhaps for some contact with the hands). Therefore, it makes no sense. =P