They are the same Vecna, there's a note in the DMG that gods like Tharizdun and Vecna have "transcended their origin on [Greyhawk] to impact the broader multiverse."
I am trying to find this on the DMG to fact-check it, but all I found was a small sentence on page 11 that states "Several of the gods are drawn from other patreons" and then mentions a bunch of gods from Greyhawk, Greece, Norway and Egypt.
But considering how some of these are altered in one way or another (including their names, in some cases), I don't see why they can't be two different characters.
And since now they have a divergent backstory, it would make even more sense to treat them as different entities.
Here's the exact line, from Chapter 5: Creating Campaigns
"Some deities of Greyhawk have also transcended their origin on this world to impact the broader multiverse. Two of these, Tharizdun and Vecna, are described in appendix A."
Then from Appendix A: Lore Glossary
"For a being known as the Chained God, the Elder Evil Tharizdun (thuh-RIZZ-dun or thair-izz-DOON) has managed to extend his baleful influence from the Greyhawk setting through many worlds of the Material Plane. He is an ancient force of entropy, the end of all things and the extinction of life. His worshipers are nihilists who seek to end all worlds by liberating their god. Tharizdun is often linked to cults of Elemental Evil (see chapter 5)."
"Vecna (VECK-nah) had humble beginnings in the Greyhawk setting, where an order of Wizards used him as a bootblack and scribe. He studied magic in secret until he amassed enough power to slaughter the order, and then he turned his efforts toward scribing the Book of Vile Darkness (described in chapter 7). Armed with that dread tome, he forged a kingdom to rule, with the vampire Kas as his lieutenant. But Kas betrayed and killed him, leaving only one hand and one eye intact (the Eye and Hand of Vecna are described in chapter 7)."
"Vecna’s evil will was so great that he persisted beyond death and eventually became a demigod of secrets and evil magic. His ambition drives him to pursue greater divine power across the multiverse."
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u/Max_G04 DM (Dungeon Memelord) Nov 09 '24
But one is Vecna from the Greyhawk universe, while the other is Vecna from the Forgotten Realms Universe.