i read somewhere(he we go with gossip and speculation)that the idea that vicki might have been elizabeth's daughter was not even part of the show's original plotting.it was noted during auditions how closely alexandra moltke resembled joan bennett and then it just added another layer to the storyline...which of course went unresolved.
That is all very true - in the Shadows on the Wall bible, Paul Stoddard was intended to be Vicki's father. However, many, many things changed from the way the bible is written (two of the biggest are perhaps 1) that Bill Malloy was intended to be a central character who would be deeply involved in the entire first year of plots, and 2) that Roger was to be killed off), and those elements were changed long before the show went into an even slightly supernatural direction. And you don't have to worry about gossip and speculation because it's actually a fact that once the resemblance between Alexandra and Joan was noticed, it was then decided that Vicki would be Liz' daughter. Joan herself had gone on record more than once to say that she was told that Vicki was Liz' daughter and that's how she played the subtext of their scenes together.
It's also true that there are many twists and turns in DS' first year that make it seem like all manner of other resolutions to Vicki's parentage could have been possible. But then it's simply the nature of soaps to throw in all sorts of red herrings to keep the audience guessing - and to potentially even misdirect them.