I had always heard this as RS saying that they had planned on addressing Vicki's parentage, not that they had already started the arc. I had no idea they had started down that path. I, like you, don't recall anything on air that suggested this direction. I wish I were able to do some viewing to see if there is anything at all that might lead one to think that. I imagine the perfect time would have been when Vicki was readying to marry. Though by that time a reveal by Liz would have seemed a bit absurd.