Vecna and the Mind Flayer reside in the Upside Down, with Vecna living in the alternate Creel mansion and the Mind Flayer in the large overarching presence.
Vecna's killings reveal a major question about the Upside Down and the Mind Flayer, and that is why Vecna is now opening gates after having been in the Upside Down for years. It would take a long time for Vecna's curse to develop if she were to become the new ruler of the Upside Down. Vecna is the only one who has had any significant time in both dimensions. Vecna has a connection to the Right Side Up.
This discovery leads to theorizing that Vecna is the Mind Flayer's "five-star general" compared to the "foot soldier" Demogorgon. The tendrils, vines, and creatures that serve as a hive brain for the Mind Flayer are found throughout the Upside Down. Vecna is the most humanoid monster in the show. Vecna is a great asset for the Mind Flayer if the monster isn't working on his own agenda. As a way to be more covert, the Mind Flayer could have used Vecna. He can open small gates to the Upside Down if he makes a kill. Number One exhibited an evil repulsion against the human race, which would explain why Brenner wanted to reduce the experiments but still keep an eye on him. As powerful as Mind Flayer is, his ability is mostly in taking advantage of his resources and not opening gates.
Vecna and the Mind Flayer seem to share a connection, but how connected are they? This time difference is explained by the fact that Vecna is working against the Mind Flayer.
Vecna and the Mind Flayer are allies. The test subjects were massacred by Number One, so Eleven banished them to the Upside Down.
The season 4 episode 7 reveal is that Vecna is the Number One. There is a connection between the Mind Flayer and the Upside Down. In the fourth season of Stranger Things, he begins killing teenagers in Hawkins, first preying on their trauma and then psychologically manipulating them. He was corrupted after years of living under the realm's influence. Henry was turned into an experiment test subject after he murdered his mother and sister. It would be unrealistic to not be connected to Vecna. Joyce destroyed the Soviet gate. Number One, whose real name is Henry Creel, had powerful psychic connections at an early age.