If your vibrato-equipped guitar seems to always go out of tune or if a string breaks almost every time you are in the middle of a ferocious solo, the problem is likely at the nut. If the nut is cut just wide enough for each string with no room for it to move once it's in place, the string cannot easily slide back and forth when you are using your vibrato. The problem is worsened if you move up to a slightly heavier string gauge. Where there's a problem, you can bet someone will come up with a solution. Graphite, which is essentially the material in a number 2 pencil, works well as a dry lubricant, allowing the string to slip across the nut, helping to eliminate both string breakage and tuning problems.