Lo and behold, who happened to be in the audience but guitarist extraordinaire Andy Timmons. Timmons—who has recorded several albums on Vai's independent Favored Nations label—was in the audience as a fan.
Luckily, the cameras were rolling as Vai spotted Timmons in the front row. As Vai plays the final section of his "Fire Garden Suite" from 1996's Fire Garden, he makes his way to the front row, takes off his beloved Ibanez JEM guitar and hands it to Timmons, who picks up right where Vai leaves off. The crowd—who you can see incredibly well in this powerful video—goes wild, as does Vai.
After shredding for more than a minute, Timmons hands the ax back to Vai at the 1:11 mark. It's a nice moment that's certainly worth checking out.