Where can I find scaled that like Kirk Hammet and Steve Vai use? Also, how do I play in a certain key? thanks.
Lol, they dont just use one scale. I have no idea about steve vai, Kirk usually uses the E minor pentatonic scale, and chromatic scale for some rhythm parts. To learn to play like them though, i'd suggest learning the Major scale first, there are 5 different fingering patterns, you can find them around UG or on the internet, memorize and practice them. The diagrams should say what key its in, and there are certain "root notes" that the scales start on, which determine the key. Id suggest looking at this


it should teach you some stuff, and once you have that, learn some solos from them and you can slowly figure out how to play like them, however you shouldnt try to copy them, adapt to your own style. Hope that helps a little.
As said, they obviously use more than one scale. I think i remember reading though that Vai's fav scale is the lydian mode. Not 100% sure though.
try to transcribe ,

I know it's a bit hard to do with Vai and Kirk but any how try to do it with the slow and easy parts they play .
Practive learning C major in 5 positions. 1st 2nd 5th 7th 9th (Starting note on 8) and 12th. I've done this with my teacher. Learn a new scale every like week. Practice them all. And when your ready, pick a random note on the fretboard. Say its with your 2nd (middle) finger, you just play the scale the you played with you middle finger (7th position.) There you go, your playing a major scale.