Vergil could have just banged a random girl just to use Nero as a way to get power sort of as a Heir.I'm still more likely to go with that he's a clone of Vergil - made by Sanctus for the purpose of powering the Savior if Dante didn't pan out.
Vergil just seems too cold and apathetic towards humans to actually go and boink one.
Man, that was ages ago, but I'm pretty sure I did not call Vergil the younger twin, but that Vergil in DMC4SE is younger compared to the one in DMC3, and certainly younger than Dante in DMC4.Vergil's actually the older twin.