Interesting, thanks. But this raises an interesting question... the Baldur's Gate book tells us nothing about that, just that Arkhan has the Hand. Icewind Dale makes Vecna a contemporary of (or older than) Netheril. The Exandria book is actually pretty vague about when Vecna ascended to godhood, except that he's the newest god in the pantheon. And the 5E Ravenloft book notes he's gone between multiple worlds.
Wizards appears to have left themselves a few different ways of handling Vecna's backstory in 5E, none of which technically contradict any of the 5E books:
- One Vecna, from Greyhawk, visiting multiple worlds (also works with older-edition lore, but not Critical Role)
- One Vecna, from Exandria, visiting multiple worlds (also works with Critical Role, but not older-edition lore)
- One Vecna, from an unknown world, visiting multiple worlds (safe middle, broad-strokes)
- Different variants of Vecna in different settings (Into the Vecna-Verse?)
That Vecna has also meddled with something that can basically create retcons may be relevant...
EDIT: Just realized that by the new canon policy, Critical Role isn't canon for D&D 5E; it's not from a 5E RPG book. So only what's in the Exandria book would count.