(Hopefully this is the right place to post this.)
So, while reading WoR, I fixated on the wording of one sentence... this is what came of it.
Theory: Vin is Iyatil.
Evidence:
“Vin rode on the seat directly across from him – her legs tucked up underneath her...” Mistborn, page 149
“Vin sat in her overstuffed chair in the conference room, legs tucked beneath her..” Mistborn, page 200
“She still wore her dress … though she sat in an unladylike position, legs tucked beneath her.” Mistborn page 320.
(I can't find my copy of Mistborn, but I found those with a quick google search.)
“Iyatil sat across from Shallan, legs tucked beneath her, watching with beady, masked eyes. The woman wore simple trousers and a shirt, such that Shallan had originally mistaken her for a boy that first time.” Sanderson, Brandon (2014-03-04). Words of Radiance (Stormlight Archive, The) (p. 757). Tom Doherty Associates. Kindle Edition.
Flimsy evidence, yes, but Brandon Sanderson seems to be the type of guy who chooses his words carefully.
More circumstantial evidence:
Iyatil carries a dagger; Vin, as a Mistborn carried glass ones, so her non-allomantic weapon of choice would likely be a dagger.
Iyatil is tiny and dresses in simple trousers and a shirt... like Vin.
It's been a while since I've reread Mistborn, so I can't say much for personality – Vin changed a lot throughout the series, so I'm going of vague memories here – Iyatil is mostly quiet – she definitely speaks before she speaks. She's very cautious. Seems to be the way I remember Vin.
But wait, you say. Vin is dead.
Yes, and Sazed was unable to bring her back to life. But, there are approximately 300 years between the Mistborn Trilogy and Alloy of Law. The events in the Stormlight Archive occur during that 300 year gap. It's not unreasonable to think that Sazed may have found a way to bring her back sometime after the close of Hero of Ages. Also, if Demoux is a worldhopper, why not zombie Vin as well? The Ghostbloods don't really seem like the kind of organization Vin would be a part of, right?
Well, we don't really know a whole lot about them... maybe she's on an infiltration mission?