OP, i think you got this wrong. Vader says that effects do not get triggered if they are based on a bonus turn, or a TM gain.
Saviour is triggered based on health, not on bonus turn. If a unit reaches 1 hp, the effect gets triggered, and the unit takes a bonus turn.
As of now, there should be a coinflip between Vader and the saved unit to determine the next turn. To me, this seems to be WAI.
Now that being said, one could argue that saviour is actually about gaining a bonus turn and everything else is tied to this effect. I for one would heavily disagree with this idea, but i have seen some very stupid arguments on this forum recently. If we accept this idea just for the lulz one could argue that savior, as a bonus turn based effect should not trigger while Vader having the merciless buff.
I think this is a twisted and incorrect idea, and savior is a health-based effect, and should trigger during the merciless massacre.