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.