Okay a list of possible settings starting with settings that will have appeared in 5e/RD&D by the time this adventure appears.
1. Forgotten Realms, this one is not even in doubt.
2. Greyhawk, with Vecna invovled almost certain to appear.
3. Eberron, I'm leaning towards yes, but far less certain then the first two.
4. Ravenloft Domains, almost certain, Vecna was a Dark Lord at one time.
5. Exandria/Wildemount, a deal will have to be struck with CR, but Vecna is worshipped there too.
6. Witchlight had a couple of League of Malevelence figures in it so Witchlight/Domains of Delight are possible.
7. Spelljammer, perhaps, but what part of Spelljammer?
9. Planescape, a given because they set up Planescapes adventure as connected to the Vecna adventure, maybe multiple planes & Sigil.
10. Radiant Citadel and it mini setting, I'm almost certain some will appear in the adventure.
11. Trickier territory here, alot less certain, but possible if part of their goals for this adventure is highlighting all their 5e settings (and possibly well love pre 5e settings as well), but Theros, Phyrexian free, is a possiblity, I mean its been placed in the material Plane and does refer to the Abyss in its MOoT. Hope the Phyrexian free D&D versions of the MtG settings do make it in. They deserve a place where they won't get wrecked.
12. Strixhaven has so much D&D, none MtG lore in it in SCoC that is at least a moderate possibly.
12. Ravnica, simular logic as Theros, but weaker ties in GMGtR to the D&D cosmology, although there are some in it.
13. Darksun, hasn't appeared in 5, but possible.
14. Dragonlance very likely.
15. Birthright, maybe, but less likely.
16. Mystara same birthright.
17. Obscure pre 5e settings like Ghostwalk, Jakandor, Council of Worms, possibly, but very unlikely.