I’m vexed by “Vox Lux.” On one hand, I love what it’s about; on the other, I’m not sure what I think it’s about is what it’s about. Make sense? Neither does actor-turned-filmmaker Brady Corbet, whose objective is discombobulatingly vague. So where do I begin?How about with what I got out of his “Behind the Music” dive into the life of a teenage girl whose brush with tragedy leads to Britney Spears-like superstardom.