It's no problem. I haven't discussed most of my theories in years so I'll explain what I mean. Also please note, I am not speaking in absolutes. I have multiple conflicting theories. Any one of them could be right or wrong, they're just fan theories after all. (this is a doozy but I tried to keep it short)
So, for the most part, I believe in two theories surrounding Will's kidnapping.
Scenario A is Vecna scanned the entirety of Hawkins as soon as El ripped open the mother gate and found Will by coincidence and started scheming then.
Scenario B is Vecna/the Demogorgon found Will when they crossed paths on the street that fateful night and Vecna immediately adjusted his plans to include Will.
(I also have Scenario C in which Will was mistaken for El because of some of the reasons below and Vecna didn't know/realize how useful Will was until Will "changed" the Upside Down to look like Hawkins the moment he entered it. But this relies on the "Will 'created' the Upside Down via creation powers by accident" which is a whole nother can of worms)
First, I should explain this relies on a few other theories such as "shiny" people and the idea that the Byers have always had latent abilities, never to be awakened or discovered under normal circumstances.
The Duffers take a lot of inspiration from Stephen King. So much so, I believe they're directing a new movie/tv show adapting one of this books which got a cameo in ST4.
In many of the Stephen King books, there are people who exist that have a certain shine to them. (hence, "The Shining"). Here's Screenrant articles about it. They're short and sweet and get the point across.
These people have varying powers and abilities and ST has been referencing them from the beginning. For example, El's story is very similar to Firestarter in some ways. They even released an official poster referencing it.
As college students, Andy McGee and Vicky Tomlinson participated in an experiment in which they were given a dose of a low-grade hallucinogen called LOT-6. While the other participants suffered terrible side effects, the experiment gave Vicky and Andy telepathic abilities; Vicky can read minds and Andy can control others to do and believe what he wants, though the effort sometimes gives him nosebleeds, limiting this otherwise very strong power. Now married, they have an eight-year-old daughter named Charlene "Charlie" McGee, who has pyrokinetic abilities (the power to control heat and fire) and can also see the near future.
Taking in all of the above, I believe the Byers, namely Joyce's family, could very well have some "shine" too. Not as much as El or Vecna or Kali or any of them. I believe the Byers just have a supernatural like intuition. I don't think it's a mistake that Joyce has been right every single season when she feels something is wrong. She might not know exactly WHAT is wrong and even starts to focus on the wrong things, but deep down, it's as if she can sense something is not right.
After everything Will went through, which was not natural and normally would not happen the average person (being kidnapped to an alternate dimension, having a tube shoved down his throat and having to be resuscitated. Later getting possessed and then exorcised), I think this intuition has evolved into something more tangible. It's no longer a just a gut feeling. Will literally feels when something is off in the form of goosebumps down his neck.
As one of the screenrant articles said, some people only have a little "shine" (Joyce. And Terry.) while others end up with a lot (El. And Will, by my theories) I feel having powers it's a natural occurrence in their world that usually goes unnoticed until crazy shit happens (like the plot of ST). It's usually something small. As examples of this:
Strong Intuition // Detecting unnatural evil things
Strong Intuition // Future-sight and predictions
Strong Empathy // Telepathy
Strong Empathy // whatever the hell Henry can do by absorbing peoples bad feelings to get stronger
etc etc. Essentially, the concept of normal real life stuff spiraling into something powerful.
The fact that signs of someone or something being on the other side appeared at Mike's house prior to Will going missing lead me to speculate that meeting on the street wasn't when Will was "chosen". Maybe in the time between El opening the gate and Will going home, Will had already been hand picked by Vecna and he was just waiting on his opportunity to take Will. If the "shine" is real, then surely Will would stick out while Vecna was scrying the town.
For Scenario B, it's the same concept as Scenario A and I think it's the most likely option.
In Scenario B, Vecna didn't see Will until the Demogorgon saw him on the street, but that's all he needed. If people with "shine" can detect others with it, then Will would immediately stick out to him.
Yes, the Demogorgon might've been out there looking for El, but if another opportunity literally crashes into you, why pass it up? Even if the moment they crossed paths was a random coincidence, nothing following that was an accident or coincidence. In my opinion, Will almost instantly became part of his plans and worth chasing on the street that night.
With all that explained, is my perspective easier to understand? I truly don't believe Will or the Byers need prior connections to the lab or Henry/Vecna for this to make sense. Trying to make too many connections between the Byers and the lab prior to November 1983 is good writing because it isn't necessary and is just a bit overkill.
All of Henry's scheming could've happened the day the gate opened. Any of his plans could've changed on a whim at any time for any reason (like for example, finding another kid with potential just sitting there out in the open)
While I think Vecna has a big plan he's been working on for years and years, I don't think he's planned out every single detail or possibility. He's made mistakes too or had to adjust because something changed.
A big example of this is El closing the original mothergate, causing Vecna/The Mind Flayer to find a new way to open gates. Or heck, I'm sure he didn't plan for them to figure out how to exorcise Will (maybe he didn't even know that was possible) so whatever plans he had for Will in season 2 and beyond had to be adjusted as well.
Henry finding Will and deciding to use him the day El opened the gate, whether he found Will at Mike's house or on the street, makes plenty of sense to me.