with that rig and guitar, you can pull it off.
Basic settings for a Mark (be it a 2, 3, 4, etc) are the following:
Vol 1=8-9
Treble=7 (pull)
Bass=1.75-2
Mid=4
Master 1=4-5 (pull)
Lead Drive=5-8 (pull) (gain to taste)
Lead Master=3-5
Presence=2-4
Engage the graphic EQ with a slight V curve. These will give you the basic Mark4 sound everyone raves about. After this, use your ears to judge how far/close you are to Vai's sound. I would recommend a few things: first reach for the mids...vai's sound has lots of smooth mids. Engage the neck pickup and roll off a small amount of volume and tone. And finally, try using a compression pedal and a boost pedal of some kind (like a tubescreamer).
Oh, and of course, lots of reverb and delay!
