No, the kids in the show name the monsters they face after the villains in their D&D game. The monsters don't resemble the D&D villains, they just share the names as the kids needed some way to refer to them. They aren't actually Vecna, Demogorgon(s), and a mind flayer.As someone who hasn't watched the show but is very aware of its ties to D&D, I was pondering something in regards to Vecna being a villain in the current season.
As D&D is a tabletop game in the universe of the show, were Gary Gygax and Dave Arneson aware of the various monsters in the series in some fashion? And thus based aspects of D&D off of their experiences? Also, did the protagonists at any point go "if anyone knows how to best kill Vecna, it'll be the writers of D&D" and attempt to contact TSR offices? Apologies if these questions are well-established ground, but they are what comes to mind first and foremost when I hear about an iconic D&D villain being an antagonist.
(In fact, demogorgon is a type of creature, not an individual; and the mind flayer is a singular creature, not a type).
Vecna
A demogorgon
The mind flayer
A demogorgon
The mind flayer