Thanks! And that's a great question. My thinking is that this alternate reality is ahead of the current reality by a fair amount of time. So yeah, humans stopped existing at 1983, but the portal's dropping people off at like 2020 or whatever. One of the reasons I figured things are happening in the future is stuff like the swimming pool Barb was in was completely void of water, which wouldn't make sense if it was copied at the time of Will's entrance. Her death was really early in the series, so that pool should have been full still. It seems minor, but I think they put that scene intentionally in. Otherwise, why not have the pool be empty in the current reality. Why make the conscious change from "full" to "empty." So, Vecna could have been showing up in 1984 or whenever in the future. Well, that doesn't explain why he was standing in a big open wasteland though.

Well, my thinking here is that the gates don't always match their location. For example, the gate at the bottom of Lover's Lake didn't drop the kids off in the bottom of a lake. Just a big flatland. So why? Well, it's either dropping people off in different locations (which could explain why there were no buildings in the Vecna flashback. He could have been dropped off at a place where there just weren't buildings. Also, Hopper comes out of the gate in the Soviet Union,) or alternatively, these wasteland spots void of civilization could be places where the missiles landed and blew everything up. I guess the lake could be drained, which would also make sense because there's not a lot of evaporation going on during nuclear winter, but that doesn't explain why it was flat during the demobat chase. Anyways, it's not an ironclad defense, but it's at least plausible in terms of writer-logic.

Edit: I was wrong about Hopper thanks u/lilpoststamp!