So Vergil was depowered after he fled from his fight with Dante?
That doesn't sound very logical and in VD his hair isn't slicked back and tidy as usual, it's a ruffled mess.
Did he run into some other demon who decided to really screw him over?
Anyhow the depowering business can be due to Dante stabbing him with Rebellion and pumping him with his red demon aura (cue sexual puns) and in turn resetting his powers back to square one.
But maybe he didn't just get reset, seeing how Dante is the stronger of the two by a wide margin perhaps getting stabbed awoke something in him.
Brace for Vergil losing to the illusions, going insane and turning into Nelo Angelo at the end of VD.