Vivian and her son, Stefan, hadn’t seen each other in over a year. They both had been portrayed by different actors at that time. She said to him, "I’ve changed, haven’t I. But you’ve changed, too." Neat way for the writers to acknowledge the previous two actors and the current actors.
No idea why they needed to bring Vivian back if Louise wasn’t going to be in the role. Too iconic of a character to have a new actress (no matter who it is) take over.