So the most important question is are dreamgirl and cave thing the same vampire or can the cave thing influence your visions? (Like you both are descendents of dreamgirl and able to see her)
Dream Girl and Cave Monster (in her "real form")
could be the same vampire, but I'm not convinced. There are several differences in facial features (
especially the eyes, and the mouth a little bit), although those could be due to an "enraged" form or something along those lines.
Still, though, Dream Girl doesn't beckon for the MC
to find her now. (Did she ever? I can't remember from the very early game.) What would be the motive for reaching him in his dreams if she doesn't try to get him to come to her? Maybe she wants the MC to weaken somehow through nefarious acts (
diablerie only being the most recent one). But that's only assuming that those do, in fact, weaken his resolve so she could call him to her again and he wouldn't resist this time.
We don't know enough to say for sure. We don't know if
diablerie would make him more manipulable. We don't know why she wouldn't be trying to wear him down steadily by more frequent visions. We also don't know why some vampires (like the Nos or the MC) got lured to the Cave Monster but
only the MC (AFAIK) has had Dream Girl visions. Everyone else seems to be like, "What are you talking about?" I feel like there's a motive missing for the difference in their behaviors if we assume they're the same.
As far as being from the same ancestor, I think that's also a bit hard to determine. We only know that both Cave Monster and Dream Girl are ancient / obscure enough that nobody seems to know anything about either of them (other than direct experience at the failed mission or in the MC's mind). I think it's more likely that the MC and Cave Monster both are somewhere descended from Dream Girl. At least, that seems more plausible to me than that they are the same vampire.
There
are many strong similarities between their faces, despite the differences I pointed out before. So I think being a descendent is at least reasonably likely. But again, we just don't have enough information really to know. And, like I said, maybe her face has different forms depending on circumstance. We've already seen for sure that Cave Monster has multiple appearances, depending on where / how she's shown up.