Natalie Portman shot her part in only 10 days.
Rooney Mara was attached to the lead role when the project was announced. After the project was put on hold, Mara dropped out and Portman replaced her.
The director claims that "Vox Lux" means "voice of light" in Latin. The correct Latin translation would be "Vox Lucis." In Latin, a word's ending depends on its role in a particular phrase. "Vox Lux" is simply two nouns, "voice light".