I think Vergils abilities were evolving like Dante's were and the shadow was supposed to suggest that sooner or later, he would acquire the Devil Trigger that Nero was supposed to have had. I mean the Sheath is evident enough right that
Take a look here...this is Vergils normal less evolved Devil Trigger...
and then his Evolved Devil Trigger which even though Nero was supposed to have had it!? It actually belonged to Vergil due to Yamato's influence
Vergil's actual evolved Devil Trigger...
And all of you saw the shadow right? So with that in mind is even more evidence that was supposed to have had a much bigger part in Devil May Cry 4, but due to (c)Capcoms stupidity in cutting the budget, and not finishing up in a proper manner decided to cut most of what was supposed to have been and released DMC4 early and way ahead of schedule.
Here's some more...
Look at the middle top fold you see a full body incarnation with legs an all that shindig, and on the right bares reseblance to Nero Angelo < Yes that was the correct spelling...it's annoying that Capcom never went with that look.
Even though it says Nero...if you look at some of the photo's below him it shows Vergil, I mean only Vergil has the classiness to look like that, and the swooped back hair is his signature daddy Sparda look.
Below him are some Vergil concept Art, and if you look towards the top left you see Vergil holding his signature Devil arm Yamato. Some Dante concept art too in the middle stating that all three characters were playable and were meant to be in the finished game.
Damn I'm absolutely exhausted...
Have a great end of week day everyone.