RESOLVED !!!
I had been researching this bug for days. I had tried every suggestion I had read to fix this. I had no luck with any of it. I had triggered the Vlaakith scene in previous play-throughs, and could do so by loading those saves, so the only variable had to be something different specifically about this play-through.
The problem was Volo's Ersatz Eye.
In other play-throughs I did not let Volo replace my eye. In this play-through I had allowed him to do so. I loaded a save from before the eye surgery, rushed back through to the Inquisitor scene, and Vlaakith triggered properly now that I did not have the magical eye.
I hope this helps others who have been frustrated by this bug.
