Good quality strings and tremolo system
ProTone Pedals: Attack Overdrive
Fractal Audio: AxeFX 2
Engl: Fireball 60
Zilla: Fatboy 2x12
Carvin: DC700
Carvin: Vader 7
Schecter: KM-7 MKii
Schecter: Banshee 8 Passive
Jackson: DK2M
i am not a fan of steve vai by any means(i am not saying he isnt a good guitarist, dont jump down my throat), but he is a 100% professional guitarist so he knows how far he can push his equipment before something breaks.
my 6 best friends:
Ibanez Artcore AF75
Schecter C-1 Hellraiser
LTD H-207 7 string
Ibanez Acoustic
He has a locking tremolo system with a cavity routed behind it, so he can pull up and down. I would assume he also changes strings quite often (as all pros seem to do) so they don't break during the gig, although that has happened while he was recording Alive in an Ultra World.
He's got a double locking trem system. Locks at the bridge and locks at the nut.
The strings are always fresh....always!
Vai does indeed get out of tune on occasion. (and break strings)

Very simple... he's got techs who's job it is have all his guitars ready at all times. They'll just bring him a diff. guitar. And constantly work back stage as needed.

On top of that... it must be nice to have Ibanez do whatever you say.