The trilogy was originally two separate works- Mistborn, which had the magic system, and Final Empire, which had the basic concept and setting. Sanderson, dissatisfied with both of them, put them together and came up with something he liked a lot more.
Vin also went through several concepts. The original Vin was actually male, but got genderflipped when the character just wasn't gelling. The next concept had Vin already being the leader of her own crew when Kelsier recruited her- this was scrapped because the "weak person discovers her own power" aspect of Vin's arc loses a lot of punch if she's already starting from a position of comparative strength. The version immediately before the Vin who made it into the published trilogy was essentially her canonical self, but much more critical of the people around her, even doing a running mental commentary of everything her crewleader, Camon, was doing wrong in her first scene. This was cut because it made Camon look incompetent and Vin more than a little obnoxious.