i guess the reason that i've always been resistant to the "liz is vicki's mother" theory is the age thing.
if we are to assume that the age of the characters is close to the age of the actors who played them then liz would have been in her late thirties at the time of vicki's birth...not some young frightened girl who found herself "in trouble" but a mature,wealthy,married lady.so what's all the fuss?
i also like to think that liz was a woman of enough character(yes,i know these are fictitious people)that even if she did find herself in a situation where she bore a child that for whatever reason she was not able to raise herself she would have found a better solution than sending the poor thing to a distant orphanage and sending a measly monthly(weekly?)check(was it for $100.00?i haven't seen these episodes in ages).
that said it apparently was a private decision on grayson hall's part to have her character(julia)fall in love with barnabas as a point of motivation for everything she was doing(with thanks to julia99).this added alot of nuance to the way things played out.i can certainly see how joan bennett acted on a similar instict because her attitiude toward vicki was certainly maternal even if that was never spelled out in specifics.
like i said it's been a long time since i watched the early episodes but i vaugely recall liz mentioning a butler named "hanscomb" who i think was supposed to be the father of the subject of the portrait "betty" who so greatly resembled vicki...perhaps there was some "upstairs,downstairs" type action going on at collinwood and betty was vicki's half sister.