Darth Vader can't kill retired Jedi Ahsoka anymore than he could kill Boba in the EU. Ahsoka can't kill Vader either as long as she knows he's Anakin, though she wouldn't know at first.
And that makes for a really good story arc in Rebels!
An older jaded Ahsoka bent on murdering the living demonic hell daylights out of Vader for all of his war crimes and cowardly betrayals, and then at the very end finding out Vader is Anakin. She ends up letting him go as he was just too pathetic and too sad for her to finish him off. She walks the balance between dark and light in her pursuit in trying to chop Vader into tiny little pieces. Vader humiliated as all hell, over his former student letting him go for being such a sorry sack of darkness then increases his efforts in hunting the remaining Jedi pretending that Ahsoka didn't just finish sonning him. Qui Gon's spirit interferes as well, as Yoda isn't the only Jedi he reaches out to.
This is where we will get an inside look at how truly pathetic a mass murdering tyrant truly is. Being a bad *** comes at a very high price.
You know I recall actually reading a Fan Fiction story few years back that was similar to this on Deviant Art. Mind you it was QUITE graphic ,disturbing, and well...Adult, (There was a VERY revealing outfit she was wearing and Ahsoka did some pretty ****ed up stuff to get under Vader's skin) so it might put off a lot of the more close minded reader, but it's sort of similar to what you proposing. Oh and Starkiller was in it and he got his butt handed to him by Ahsoka. I also recall Obi Wan was in it, on his way to becoming Old Ben, he was trying to persuade her to give up the path she was walking and how close she was falling. It's been a while so I might have forgotten everything.
Now I know there is no way in the Spice Mines of Kessel we're going to see something like that, which I'm sure a lot of you are grateful, but still it was kinda similar.