In my experience Visas Marr sneaks on the Ebon Hawk the moment I land on any planet after I leave Telos.
It might be possible that it will be 25% Dark or Light, but never checked that, I am quite dedicated to either side when I wish so might be possible.
Visas does come after the cutscene where she leaves the Ravager. It is triggered when you are 25% to light or dark side.
So if I go throughiut the game without Visas, make it to the Ravenger, I can still fight Darth Nillius exactly the same way? Will I see her in her chambers?