the official position of Dan Curtis Productions today is that Elizabeth was Vicki's mother (i.e., this was the direction Mark Rainey and Elizabeth Massey received in writing their official DS novel, and I believe this was the position of the recent updated play presented at one of the recent festivals). However, not everyone agrees that this was always the intent
Well, no, it wasn't the intent in the Shadows on the Wall bible - but that was written before production of the series actually began. However, it's not quite true that it wasn't the intent once DS went into production. As I've previously posted:
...
Joan Bennett had commented that she was told almost from the very outset that Liz was Vicki's mother and that's how she always played their relationship, and ... DS producer Robert Costello has commented that, after they hired Alexandra Moltke and saw her resemblance to Bennett, they decided to deviate from Art Wallace's bible and make Liz Vicki's mother.
and the issue was never resolved on the series itself.
But as I've also said, the great thing about fandom is that it's possible to debate and deconstruct all the unresolved questions on DS. And we should never refrain from engaging in debate simply because it seems as if a lingering question was resolved in a way other than during the course of the show (like Bennett and Costello's comments). Unless there's an explicit and definitive answer to a question in the actual episodes, then an issue is and always will be up for debate.