Vicki saw Josette's ghost herself when [spoiler]Matthew Morgan had kidnapped her and Josette's ghost rescued her[/spoiler] but the ghost did not speak then, I don't think. This change in the ghost at just the time Maggie has disappeared would put up warning signs all around me IF I thought it out for a length of time.
There's a precedent for Josette's ghost to speak, and Vicki was aware of it. After the incident you mentioned in the spoiler box, Liz recounted a story to her about her great aunt who, as a child, was lost on a remote part of the beach in the Winter. But a woman in a long white dress appeared to tell her to not be afraid (and may have caused her miraculous rescue as well). Vicki and Liz agreed that the woman must have been the ghost of Josette.
Beautiful screen grabs, MB!