Speaking as a 1E vet, the Greyhawk mythology really wasn't intended as a unified whole -- a lot of the stuff in the artifact section of the 1E DMG (such as Vecna and Kas, or the Fables of Burdock) is only mentioned there, and never again. Kyuss, who was first mentioned in the original Fiend Folio, had no ties to Greyhawk in his first appearance, either.
Yes, one of the oldest D&D settings is Greyhawk, but that doesn't automatically mean everything old is of Greyhawk, either. I think you could quite comfortably set the Age of Worms in any old school setting like Blackmoor, Mystara (which was old school well before it was even the Known World) or even the Wilderlands of High Fantasy.
AoW is nice in that it ties together a bunch of great old school stuff, and obviously that fits like a glove into Greyhawk, but I disagree that classic = Greyhawk and nothing else.
Vecna, for one, belongs to the ages and not to Greyhawk or, I'd argue, even just D&D any more.