Hmm, although I like Ellen Paige, I really don't see her as Vin, at all.
Vin - Mara Rooney
Kelsier - Paul Bettany
Most of the rest should be lesser known or unknowns. I've seen how well that worked in Lord of the Rings, and now in The Hobbit as well. Too many famous actors make it top heavy and ends up like the horrendous Ocean's trilogy.