Well, it wasn't as clear-cut as that; the story as told to me (I'm probably getting some details wrong) was that Mother was talking to Vilma, selling her on the idea of doing Sister Stella L. The clincher was when she called Nora (or Nora called her) and offered her the role, to which Vilma said she'd do it. It could have been a ploy to land Vilma.
Mike and Nora? I don't know, Mike does like understated acting and Nora's best when she's understated or even silent, but when you watch Mike's movies you don't get the sense that it's all about the stars, or the characters, or even the movie. It's all about Mike's vision of the world, as filtered through what story he's telling at the moment. The performances (excellent tho they may be and I think Vic Silayan in Kisapmata gives the greatest male performance in all of Philippine cinema) are secondary.
That's not a negative comment, it's an observation. It's something almost unique to Mike's work. In Bernal, Brocka, O'Hara, Ad. Castillo, Aguiluz, lumilitaw ang performances, ang mga characters; in de Leon (and possibly Raymond Red), yung concept ang primera.