Quote Originally Posted by Arkat View Post
This might be somewhat helpful.

https://en.wikipedia.org/wiki/Savage_Tide

It shows the Isle of Dread was introduced into Greyhawk in the 4th installment (January 2007) of the Savage Tide AP. If the Isle of Dread can make it into Greyhawk, I see no reason why Vecna couldn't make it to the Isle of Dread.
That's an interesting find Arkat. That suggested campaign path has you moving directly from the Saltmarsh series to the Isle of Dread which is interesting because Saltmarsh was very much originally set in Greyhawk, while IoD wasn't placed there directly until 5e.