Agree that Louise Sorel is the definitive Vivian, with all due respect to the other fine actresses who have played her before, Linda Dano and Robin Strasser.
Sidebar: Just a thought, maybe others share it: While I really do not want to see a Nicole recast, if they ever do one, I don't think they can consider AnnaLynne McCord now. She would be a good choice. Terrific actress, attractive, I can see her in that role if the powers that be went in that direction. I DON'T want this, but just saying. Also, if she sticks around for a while as Cat, it would be even harder to believe her as another character. I just want great things for this actress, she is a fine addition to any show, no matter the role, good or bad or somewhere in between. My two cents.