I wouldn't say it is guaranteed next attack, because it isn't, but it is damn close to every time as a drow.

As a low Dex ap it doesn't usually give you next attack, and you are generally better dual wielding and excising pebble to boulder or even crushing hand or whatever it is called. Neither of them is guaranteed either but you have a better chance.

Personally I think vault should be a hero level skill.

But I'd also make it more reliable on awake enemies. It's hard enough to pull off on an awake person what with the advance lag, so if they are still standing there, you deserve a high chance of hitting them, in my view.