Something we probably need to remember is that supposedly Vicki was simply given David's medical file - she never met the Collinses before she was hired. In fact, unlike in past TV versions of DS, the pilot gives us no idea how Vicki was hired. In the original series, a letter was sent to Vicki's foundling home requesting her services - and that certainly wouldn't work nowadays - which is no doubt why it was even abandoned in the '91 DS in favor of Vicki being hired by the Collinses' lawyer. The latter may have also been the case with this Vicki. Or maybe not because the voiceover tells us that Vicki also has her own hopes unrelated to tutoring David. So, it's quite possible that the lack of an explanation regarding Vicki's hiring might mean it was something that was deliberately left out of the pilot so as to be gotten into had the pilot gone to series. But as the pilot stands we also have no way of knowing whether or not Vicki was given any info about David's mother before deciding to take the job. Though if she was, in all likelihood it would have been that the Collinses refuse to talk about her. And despite that, Vicki still decided to take the job - and for all we know, her question about David's mother in this scene might have been her sly way to try to get to the truth about David's mother.
All that being speculated, though, what will become clear before the scene ends is that a bit of dramatic license needs to be granted to the scene because it's really not about David's mother - it's about Roger's reaction to the subject of David's mother. Having Vicki question and comment about the lack of info concerning David's mother is really a device to elicit Roger's reaction, which we'll soon see is intense. And that's something that makes all the sadder the fact that all of Roger's stuff was dropped from the script for the pilot.
