Summer Glau as VIN
Reasons: She's definitely athletic enough as she has tons of dance-background. Also, she has a very aloof way about her and is generally a very quiet person as a whole (not just as her character of River in "Firefly" but just like when she's being interviewed). I think she'd be perfect and she doesn't look out of the age range, even though she technically kind of is.
Nathan Fillion as... well. He can definitely be Kelsier. But, I wouldn't be opposed to just seeing him in the movie. He'd be a great fit in a Mistborn film.