A Question

I'm not sure whether people have asked this before or not. If they have then could someone please direct me to the entry so I can maybe find some answers? Anyway, so I watched Advent Children again, for the nth time, and I realized that I still don't understand why Vincent can't take Marlene back to 7th Heaven. In the official English sub Cloud asked him to take her to the bar, and Vincent's answer is "You know I can't." So . . . um, anyone know why? I had thought maybe it's because he's bound to stay in the Forbidden City but then later he went to Edge, so it can't be that. Any ideas/conjectures/facts about this?