I'm not surprised. But still, how? How?
And, more importantly, who? I'm not one of those people who thinks Vergil is afraid of girls, but the chances of him knocking up some hapless chick in Fortuna and then swishing off into the sunset seems far more like Dante's style than Vergil's. Forget it, I'm gonna continue to interpret "Vergil's child" as "Vergil's clone" because this is stupid.
(Although I already reconciled the age thing. We're never told how much time passes between the end of DMC3 and the beginning of DMC1. If Vergil had relations shortly before DMC3 and at least ten or so years passed since then, Nero could be an older teenager by DMC4. It's still stupid but possible).
Also I agree that that artbook looks freaking sweet. I waaaant, I waaaant.