I've got to say of all the SF vs DOA threads you've created this might be the closest one yet. We should summarize it like this:
Speed: Both combatants are extremely fast. Vega might be the fastest SFer ever, and Kasumi without a shadow of a doubt is the fastest DOA character. No argument there.
Agility: Vega has mad agility. That much is clear since the good ole Street Fighter II days. So does Kasumi. Vega may have her beat in that department though.
Experience: Vega must have plenty of experience, but IMO Kasumi may have more experience eventhough she's only 17. She's fought the likes of Raidou, Alpha 152 and has been fighting for pretty much her entire life. Not to mention she's always on the run and looking for Ninja assassins wanting to end her "traitorous" life.
Lethality(wonder if this word exists, but whatever): Vega is insane with his claws. Them claws combined with his speed and agility may him a lethal adversarie not to be joked with. Kasumi also has her blade and she's quite skilled with it and she also has speed and agility to back her up.
Come to think of it I really can't decide who would be the winner in this matchup. The only way I can see a possible outcome is if Kasumi manages to disarm Vega. Then he will be in a world of pain. But I reckon disarming him is not going to be an easy task.