Vecna can cast 1 8th level spell (and 1 7th, and 1 6th) in a day. Sadly he has no 9th level spells.Can't you figure that from the highest level spell listed?
A 15th to 20th level wizard can cast one 8th. (17th + gets one 9th)
So I guess in a way that does answer the question.
Edited to correct mistake mentioned below,
Last edited: