VI Vader is there to deal with Poe, Whisper, and Fel. At least in my meta, it's a high PS meta. Turr isn't a good choice because he's out PS'd by the other Acrs which makes his ability all but moot. Plus if you get stressed, then you can't use his ability at all.
Agreed, it has to be Vader due to PS 11 (and ATC).