I think I read somewhere that Niko's story is set over the course of a year. Deep sleeper, I guess.
If so, then we can safely go ahead and say that Mallorie was knocked up by Roman, or someone else, quite a while after Vlad died. Infact, Vlad dies so early in the story that it's quite possible the baby would already have been born had it been his. You also have to take into account that Vlad appears to be a heavy drug user, so it's unlikely that a man of his age and lifestyle could father a child.