Except that he is warping space and that requires FTL movement.
Where does it say he is warping space!?
So let's explain everything through magic? Magic is a part of their universe but I doubt it's the cause of every single feat the characters do and there are indications that show that the characters are doing these things unaided. For example, as indicated by the blurring effect, Vergil is moving at high speeds when he's using Air Tricks, same thing with Dante. Besides, if magic is allowing him to move FTL then he is still moving FTL.
Except Vergil's Air Trick is a magical, and instantaneous teleportation. It isn't him moving at high speed in a traditional sense. He's not walking, running, or jumping at high speed during his Tricks.
That blur isn't his super-fast movement, because otherwise it would be one continuous blur from A to B, like when we see Dante using his tricks. The blur is him fading out for that spot and reappearing
That's like denying that DmC Dante is ever capable of punching Mundus in the face because he needed Eryx to do it. Whether he used Eryx or not, he still did it.
Except Dante did punch Mundus without Eryx...
Ummm... If magic is involved, why does it need to collide with anything or have any ordinary physical reaction with anything else?
Because they all exist in friggin' reality, and they actively working against the laws of physics all the time when they do all their super-cool sh!t. Air obviously exists, because everyone seems to breathe, which means air molecules exist, which means if something goes fast enough, they're going to be colliding with air molecules. I'm just looking at what was shown in-game, and in-game, anything moving at such a high speed will incur aerodynamic heating, as we saw from Dante showing off after his awakening.
Capcom set that law in motion there, not me. I just observed it :/
For example, if Dante through Rebellion and used magic to speed it up, would that same heated shell be produced? Not necessarily as it would depend on what the magic was doing.
It probably would actually, unless there was a specific part of the magic that worked against aerodynamic heating, but we've yet to see anything like that. The only instance we see of aerodynamic heating is during Dante's show-off scene, and that was just him throwing his sword and then running after it with magically-enhanced strength. I say "magically-enhanced strength" because there's no way the human muslces, on someone with Dante's physique, could throw something or move that fast without some sort of metaphysical aid, like magic augmenting a person's strength.
Now, as for consequences of approaching that speed, it depends on how fast you accelerate. A gradual acceleration would cause the reactions we see normally but a sudden burst of speed would be much different.
I think you've got that totally backwards. The faster something goes, the more resistance it creates because of everything it's moving against. It doesn't matter if it's a burst or gradual because all those air molecules are still there, no matter how fast you're going. Plus, the slower you move, the easier it is for something to get out of the way.
Think of water; dive into a pool from the edge, you'll shbloop right in. However, dive into a pool at terminal velocity from a plane and the water is like solid concrete, because you're trying to move through the water so quickly that the molecules don't have time to get out of the way.
The same goes for air. And the faster you go, the harder molecules are going to collide, until some really cool sh!t starts happening; http://what-if.xkcd.com/1/
Plus, ultimately, you're forgetting the fact that even if Vergil was doing something faster than light, we would never see it happen. If he could perform FTL iaido, we'd seen him place his hand on his sword, and then something would die. We'd never see any movement.
And since we can pretty much see everything he's doing, it can't possibly be FTL. Hypersonic, maybe, but then that all comes back to the fact that Vergil is slicing at hypersonic speed, and handspeed is not indicative of overall speed.
The only time? How about everytime Dante enters Limbo when pieces of buildings break apart and start floating around in the air? Or when Kat literally shifted space using a timer filled some concoction of... stuff?
I said the only time people are affected by the gravity in Limbo. The people, the living thingers running around in Limbo, not the debris.
If physics is going to "take a break" then it's no longer physics and there's no way of objectively measuring how strong Dante really is. I mean, thanks for those small moments of "gravity" where some stuff fall while others still float in the air, but physics needs to be consistent for it to have any bearing.
Again, keep in mind that Malice is what's keeping debris in the air. Essentially, imagine Malice as a dude in the background holding stuff up by string. Everything else, though, still affected by physics.