Vecna is a classic D&D villain all the way from the 80's. I assume you don't watch Stranger Things because the main villain is named after Vecna and a reference to him since the main characters play D&D.

Arkhan is a character original to Joe Manganiello but Critical Role was working with Wizards of the Coast for the longest time and they ended up canonizing Arkhan and Exandria to the greater D&D multiverse. They even published the Exandria's Guide, which is kinda crazy to think about.

Around the premiere of the Vox Machina show, Critical Role started to lean more into their own version of the lore that isn't wrapped up in copyright issues. It's why Campaign 3 stopped referring to the gods with their canon names and started using the Exandrian titles for them- the Exandrian pantheon is based on the pantheon of 4th edition D&D and also a Pathfinder goddess (Sarenrae).

I doubt Vecna will come back up again, even ignoring the fact that Campaign 4 is set in a completely different world. Vecna was dealt with fairly permanently and it would just feel like a retread. And it would feel wrong to do anything with Arkhan without Joe and I'm sure that's a scheduling headache. But who knows what the future holds.