If you read that whole Arlen Roth thing above, it says that ARLEN actually is the one who taught Macchio as he was with him on the film set and off for 5 MONTHS. They realized that Ralph could make it look like he was playing, but Arlen was recording a lot of the music for the scenes. I think Arlen is saying that a lot of what he played was cut and played over the top with Ry Cooder's music for the film (since Ry wanted a big piece of the credit). Then Arlen got in a fight with the producers over putting Vai in the movie at all. So the producers changed the end entirely to put Vai in (thus dating the movie as Roth claims... I kind of disagree... it was saying that the Devil exists in every generation and has his hooks into whatever is going on... Vai wasn't playing hair metal. He was just gripping it and ripping it).
Then Roth claims that the end guitar scene jumps between, Cooder, Roth, and Vai all playing Macchio's part because they edited it all together. It makes sense when you hear the different sounds that Ralph's guitar is making as he's playing different styles. When he's playing slide it sounds very much like Cooder's playing. When he's playing rock stuff it very well be Roth, and when he is playing the classical I am going to jump out on a limb from what Roth said here and say that that portion is a combination between Roth and Vai. Or they had Vai just re-dub the same stuff Roth played. But there is no way that I think Vai was playing Ralph's part where he's using the bottleneck. Steve is mimicking that sound with his signature style "JACK BUTLER LADiES AND GENTLEMAN, JACK BUTLER!"
So Roth's writing linked above adds another layer entirely to what Roland and I discussed and told what we had always heard over the years about the great head cutting competition. It's interesting reading at least, and I enjoyed hearing Arlen's insights on the movie. It's a 10 minute read for anybody that is interested (and probably half as long as this post!).