Vecna and the Mind Flayer in Stranger Things clearly seem to share a connection, but how connected are they?
Throughout the Upside Down are tendrils, vines, and creatures that serve as a hive brain for the Mind Flayer.
Being connected to the hive brain through a spider-like network of back extensions, Vecna is the most humanoid monster in Stranger Things.
This discovery leads to Dustin (Gaten Matarazzo) theorizing that Vecna is the Mind Flayer's "five-star general" compared to the "foot soldier" Demogorgon.
As powerful as Stranger Things' Mind Flayer is, his ability is primarily in masterfully taking advantage of his many resources and not opening gates.
Vecna, however, can perhaps open gates to the Upside Down because of his connection to the Right Side Up.
Out of all the known Upside Down creatures (even the Demogorgon being held extensively at Kamchatka), Vecna is the only one who has had any significant time in both dimensions.
This fact coupled with his psychic abilities makes Vecna a terrific asset for the Mind Flayer, assuming that the monster isn't working on his own agenda.