I just learned from my guitar teacher (who is a god, but that's irrelevant) that Vai uses his Eventide harmonizer almost always when he plays his fast tapping stuff. Listen to Passion and Warfare, if you find a tapping part from it which has not been harmonized, please, point it out to me.
That's one thing.. Perhaps..
"If practise makes perfect, and no one is perfect, why practise? Duh.."