When does an individual human life begin? The answer to this question is easy: at conception. For if an individual human life begins at some time t after conception, then what existed before t would have to be either not an individual or not human or not alive. And the absurdity of that ought to be self-evident.
But note that the existence of identical twins demonstrates that an individual human life can begin after conception. In such cases, two human individuals can be traced back to a single zygote. As such, those two individuals cannot be numerically identical to the zygote, or else - by the transitivity of identity - they would be numerically identical to each other, which they are not. (They are two individuals, not just one.) We must conclude that at least one of the twins (or, less arbitrarily, both of them) began their "individual human life" at a point in time after the zygote/embryo split in two, and thus after conception.
It technically does not follow that what existed before was "either not an individual or not human or not alive". Perhaps what existed before was a different individual human life. (Cf. Parfit's dividing person thought experiment.)
But, technicalities aside, I take it that BV means to be responding to someone who denies that the zygote is an "individual human life". From this claim, it does trivially follow that the zygote must be "either not an individual or not human or not alive". I'm not sure why this is self-evidently absurd, however. It depends upon what we mean by "human". As used in this context, someone might speak of "human beings" in a sense that goes beyond the mere biological classification of an organism that contains the appropriate genetic material.
I don't think it's absurd to deny that zygotes are human beings. One might see the zygote's relation to a human being as similar to that of a caterpillar to a butterfly. The one develops into the other, but we can still recognize that they are distinct entities. (Even if they share a single revolution around the circle of life, their distinct forms might justify treating the different parts of the life-cycle as a different kind of entity.) For example, here's a simple argument for the zygote/human-being distinction:
1. An essential feature of a human being is that it has a brain.
2. A zygote does not have a brain.
3. Therefore, a zygote is not a human being.
Of course, none of this addresses the "real question", which BV recognizes is the question of when an individual becomes a person in the morally-relevant sense. Nevertheless, examining and (hopefully) clarifying even minor questions can be an enjoyable and worthwhile activity, as I'm sure BV would agree.
P.S. See also Infanticide & Abortion and Abortion's Common Ground.