I was wondering if there is a certain technique to get the really fast vibrato on bends like dimebag/kirk hammet. Is that with a whammy bar or just something you have to work at?
its whammy bar, flip your guitar sideways and hold onto the bar with your whole right hand and visiosly move it up in down.

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yea...but i wouldnt recommend that unless you have a couple extra spare sets of strings lying around, haha
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Both of those guys have Floyd Roses, so it could very well be that they're using that. If they're not, you won't get a lot of speed if you try a more traditional vibrato. With a normal vibrato, you bend up and then release back to normal tension. When you bend up, you release and then push back to bent note. Repeat as needed.

From there, just a lot of practice.
Ok, that doesn't look as clear as I would've liked for an explanation...
it could be a trem flutter, you need a floyd rose with quite alot of spring tension, then you play a note and run your hand along the whammy bar and flick it of the end and gives a kinda really fast vibrato.