The way I think this works is: without atium Brandon/Adonalsium can look at Vin and say "She has these desires, these instincts, these abilities, and this information, therefore she will do this." Because Brandon is omniscient about the Cosmere (for the same reason Jesus is omniscient about this univerese), his predictions are perfectly accurate. When Zane burned atium, he was tapping into this knowledge via Ruin. But, by paying very close attention, Vin saw Zane react to a move that she hadn't yet made, essentially getting a look at her own future. This changed the information she had access to, thus changing the course of action she chose to take.
BTW, I'd like to make it clear that I agree with Voidus and 213 that free will and predestination are not mutually exclusive.