This is probably a question that will be answered with a short ''yes'', but did anyone notice that when Vergil is leaving Fortuna in the last cutscene, Vergil's shadow actually looks like Nero's devil trigger? He's got those halo-like things attached to his shoulders too, it seems. Obviously, Vergil's own devil trigger doesn't look like that, so it doesn't make sense at this point in the storyline, but it's clear that this is foreshadowing that shows Vergil will become Nero's devil trigger.
As for what he says in that cutscene, ''soon they will know this devil's power'' or something, to me that implies that Vergil has gotten a plan to raise the Temen-Ni-Gru and obtain Sparda's power, which happens in DMC3. So anyone who still thinks the cutscenes take place after DMC3 somehow (even though Vergil was enslaved by Mundus and if he came back should look much older than he does in DMC4 SE), take a look at that cutscene. I think the cutscene in original DMC4 with Nero saying ''didn't figure this guy for a bookworm'' makes sense now... it wasn't Sanctus who read so many books, or even Agnus, no, it was Vergil. He was there to read about Sparda, to get to know more about him.
Anyway, I now think Nero is definitely using Vergil/Yamato (Vergil is inside the Yamato) as a catalyst for his demonic power. I am wondering, though, if Nero will get his own devil trigger at some point. Or whether or not Vergil will somehow take more control of Nero's body. After all, Vergil wants more power, so would he just stop, or would he try to restore his own form partially? Or would he perhaps be too respectful of Nero to take so much control of him?
I still think Nero might somehow have something to do with Sparda, though, considering there are so many hints to his similarity to him. Not to mention that the idea of Vergil having a child with a human sounds wrong, considering he hates humans' frailty, their lack of power. He always seemed to think it was preferable not to have any contact with humans, and he certainly wouldn't want to dilute Sparda's bloodline -- I mean, he reveres the guy. He even says in reference to the Order's deification of Sparda: ''well, I can't exactly call them misguided''. He thought humans were weak, so why would he want his and Sparda's bloodline to become weak? It just makes no sense. For all we know, Nero is like a backup plan to Vergil: if something goes wrong in DMC3, there's always Nero to continue Sparda's bloodline.