If you found out that Vivienne isn't actually Vivienne, I assume that you did find the 'second story' which is essentially the first endings minus the illusion of Vivienne being cute and innocent. If not, start a new game once the main menu CG and music changes to find 3-4 endings or so. Have you seen the scene where she hid in the closet etc.?
Anyway, thank you for playing the game and for liking it!