I think it might be simpler for Vecna to do something like:
At will: any spell on the wizard spell list 8th level or lower as an 8th level spell
3/day: any spell on the wizard spell list as a 9th level spell (total of 3 9th level spells per day), any spell on the cleric spell list 8th level or lower as an 8th level spell (total of 3 8th level spells per day)
1/day: any spell on the cleric spell list as a 9th level spell
I am not big on cantrips and low-level spells for gods unless they are casting them as 7th level or above spells. It is fine for a lich to cast the odd 3rd-level spell at a party of PC's, but it should be beneath the dignity of the god of liches to do so.
Some lower level spells might be better used as special abilities. So instead of hex, Vecna might have Mass Hex (1/day): everyone with 200 feet of Vecna that he chooses must make a X DC wisdom save or be affected by a hex spell until their next long rest.