Dave Filoni: I have always believed that Vader has no repetent bone in his body until he realizes his son is alive.
Vader is not going to be swayed be Ahsoka being alive. That is my entire issue with your theory. It devalues Luke's importance to Vader's redemption. As Filoni said, Ahsoka is not going to be igniting any spark of good. Vader is not going to be angry that he killed Ahsoka. If anything he is going to kill her because she is a living memory of his past.
Now, I am seeing this drama play out over multitude episodes. Ahsoka will try to reach Anakin. Vader will try to turn her to the dark side.
There is a huge difference between him being "Swayed" and not wanting to kill her. Like you said, he'd probably want to convert her to the dark side. If he's unable to do it he'll probably end up killing her, but my theory is that killing her isn't his ideal solution.
I'm not saying she'll redeem him. She might try however....... but I'll agree that there's no chance she'd succeed. Unless they want to create an alternate timeline - (Which could be really run as a side project).