Heh. I wonder what the reaction would if the stat block's spell section just said "He's freaking Vecna. He's a 20th-level Wizard that knows and can cast every spell in the PHB."
22 years ago I had an epiphany while running a D&D game at 3 in the morning surrounded by crying nerds during a 48 hours gaming session fueled by Jolt Cola and more Little Debbie than you could shake a Honey Bun at. I had spent hours meticulously crafting an emotional narrative about the party's favorite character and when it came time to pay off that narrative I "Broke the Rules" of the game.
Before that point most of my NPCs had careful stat blocks that basically made them into characters. Like full on wrote them up as PCs when they didn't fit the character blocks in the Monster Manual (As it was the only NPC-Book at the time and it would be a while before the MM2 or anything came out with more statblocks to run through)
I used an ability he "Didn't Have" because at 3am on the second day my head got a little loopy and I crossed class abilities in my brain and combined them into a new ability that didn't actually exist in 3e at the time. But it worked for what I needed, and it paid off everything beautifully.
And I was surrounded by nerds crying over a monkey-girl and vowing to BURN DOWN THE CITY that made her sad. Not only did they want to stop the BBEG, they also wanted to crush everything he stood for.
And it was at that point that I realized that big important NPCs aren't bound up by the rulebooks.
Vecna isn't a level 20 Adventuring Wizard. Vecna is -VECNA-.
Can he do everything a level 20 wizard can? Probably not, some of it is way too mundane. He's not level 20 in the Wizard class. He's level 20 in the Vecna class. And the Vecna class isn't something your players get access to. He's not bound to the same rules.
Once you accept that. Once you truly learn that NPCs don't have to follow the rules of character generation... you learn other rules they don't have to adhere to.
Will that piss off some gamist-purists? Of course! But it can make things -way- more interesting.