Prop Shredder (or just Shredder, I forget) with Osiris can reliably deflect any attack in the game, including Vergil's projectile slashes. I think if you can deflect one back at him, it'll cancel the entire attack. I forget, though.
Alternatively, you can just dodge from side to side on the ground.
EDIT: Wait. Do you mean when he does multiple Dimension Slashes? That's a different story, and I'm not sure if there's a way to break him out of that.
~@_@~ Why are you people so hung up on what's canon or not? - freakazaa