Totally random opinion, but all I can picture in my head for Vin is Lily-Rose Aslandogdu. Y'all have to watch the Save the Children video that stars her--her acting is unreal, especially for her age.
Knowing how long productions take, I think she would make a lot more sense. She'd actually be closer to Vin's real age, and would grow with the series. I think a lot of the other actresses that have been suggested are too adult, curvy, sexy, etc. to play Vin, especially initially. The first book strongly emphasizes how small, thin, and scrappy she is, and how easily she can hide herself. She's grown up being starved and beaten--an amazing powerful healthy curvy woman doesn't seem anywhere close to the right fit. I think this actress's sharp features and super-focused stare work really well for Vin. Plus all the hinting in the first book of Vin being like the daughter Kelsier never had--a Natalie Portman or even Summer Glau would not be close to believable as his daughter figure. I think the next closest suggestion I've seen is maybe Brianna Hildebrand or definitely Mckayla Twiggs.