Seconding the Arkham scenario, and adding that Vergil's also holding back at the beginning of the Nero fight. (Kinda like the Dante fights in DMC4, except there's that musical cue when Vergil starts taking it more seriously.) If Vergil had been in the mood for filicide, I don't think Nero would've made it. I think Nero has the potential to be as strong as the twins, but I think he needs a bit of time and growing.