You can tell Ashoka knew by the look on her face after telling the others she didn't know who the Sith was. I don't believe Ezra is old enough to know what a Sith Lord is, let alone who Vader is. Kind of surprised Kanan didn't recognize Vader at the same time Ashoka did.
As far as killing off Ezra.. I kind of had rather seen Kanan fall or at least be captured and Ashoka train Ezra instead.