At the risk of being coin-shot for mentioning her name, I think Kristen Stewart could portray Vin (physically) very well. She's got the boyish looks while still being stunningly attractive, the incredibly slim build, and short height. She also scowls a lot, and shows very little emotion in her expressions most of the time - which is Vin all over.
The downside, of course, is that her acting is mediocre at the best of times. I don't think she could really pull the character off. And that's such a shame, because physically she's perfect. Her looks with Ellen Page's acting would be perfect. I think Ellen is an incredible actress, but her face is too round to portray Vin. Especially throughout the first half of Mistborn where she's a malnourished, gaunt, and beaten girl. Hopefully they can discover a new actress who's a perfect fit in all accounts and blow us away.