Usualy for bigger bends , you should always use help from other fingers ... The strongest bend you can do is to bend with your ring finger with help from the middle , and hopefully the index (if it's not busy).
I don't think Steve Vai uses any special bending technique (considering the freting hand) , but he sometimes helps his bending with the tremolo bar (you can bend up to umpteen frets this way, no matter what gauge you use).
If you're still having problems (after a couple of months or so), you may have to sacrifice tone for a lighter gauge strings .
Don't forget about the vibrato thing "noticingthemistake" mentioned ... It can be more important than bending itself.
[Edited by SLY on 07-05-2003 at 12:53 AM]