All of the main cast in Xmen including Young Magneto
suddenly entered into a dimension that teleported them in to Twilight Universe. Being lowkey, they heard of the Vampires existence. The decided to go to Italy to pay the Volturi a visit in one big group. Let the confrontation begin.
The speed of thought is faster than the vamp's running speed. And we've seen just how fast his powers worked. In X2 he froze the entirety of the mall in a split second. And he did the same in the Oval Office I believe.
Those were humans unaware of what was going on. The vamps know a fight is going to take place and they are much faster than a human can visually perceive.
You may be right, X may stop them before he gets beheaded, but it would be close. If X did stop them, I bet he'd have a hand near or on his face before anyone froze.
By the way, I hate arguing in favor of Twilight. I like X-Men a lot more.
Them knowing a fight is happening doesn't increase their resistance to his incredibly powerful telepathy. They just know he's going to use it.
Also, we have people like Colossus, Juggernaut, and Blob who the vamps aren't strong enough to get past who can just guard him if they feel it's an issue. Or just keep Shadowcat with him and phased, or Sunspot put a ring of fire around him, or Blink put up portals, etc...
They can keep Xavier very safe without much trouble.
All depends on Xavier. Without him X-men get destroyed. With him, and the apparent foreknowledge/jump on their opponents his team has, they win. Be interesting to see what would happen if one of Havok's plasma rings hit a vamp. One of them proved powerful enough to overload Darwin.
__________________ Never take life seriously. Nobody gets out alive anyway.