Even if you suppose that the predicate ‘being a person’ is vague, it is true that, at some level of vagueness, a being moves from not definitely all the way up a textbook person to definitely all the way up texbook person in an instant. In an instant, the being moves from being the sort of thing that has hardly any value to the kind of thing that you cannot terminate with doing an extreme moral wrong. But it cannot be true that the natural properties you acquire in a single instant are sufficient to make that great of a moral difference.
(P.S. – It might help to read the whole thing.)