You might want to check out the Fulltone Distortion Pro.I don't know if it will give you the Vai/JS/JPM sound but from the description on the page,I think you can get some amazing tones from the pedal-
Allan Holdsworth to Brian May to EJ!!
http://www.fulltone.com/DP1.asp
"Coming soon to a store near you!
Not a FUZZ! The greatest hi-gain sounds you can imagine, from Brian May to Boston, Holdsworth to EJ in a little metal box -- dripping with harmonics and feel -- unlimited shades of dirty, crunchy, Bluesey, and it even cleans up well as you turn the guitar's volume knob down. Leave the pedal *on and do everything from the guitar!
NOT A TUBE-DRIVEN PEDAL (and much more "tubey" sounding than any I've heard).
Volume, Distortion, Resonance, Voicing,
Hi-End and Saturation controls -- capable of turning the thinnest brightest amp you own into a 100 lb. violin.
"