Is everyone sure that Vergil came from Yamato? I think Vergil is the reason Nero has the arm in the first place. I think the DT came from Yamato itself. It actually makes some sense if you consider the fact that Vergil gained his DT before Dante in the first place. Probably because of Yamato. The sword awoke his full potential.
I am pretty sure he had the arm before he sealed Yamoto
his dt looks like Vergil's super form in 3