There's a thin line between DMing and playing the PCs yourself, I know I find it hard not to point out the various times my players don't notice x or y ability at the right time.
Sigh... Yes, it is very hard to be unbiased. Often times I find myself saying (to myself LOL) "Why did they DO that and not do THIS!?"
Of course, even when I try my best to not act with DM knowledge, I understand I have insights because of it that players don't and so they might not think to do something or use some feature.
And yes, players DO also forget features of their PCs, but in this case I think a lot of it is because we are doing a:
"
WHAM! Here is a 20th-level PC which you didn't play from the ground up but I still expect you to know
everything your PC can do and play them like a PC with dozens of hard adventures under their belt."
This fight is all about the lair
While the lair plays a HUGE role here, another aspect is I am gaining experience playing Vecna and utilizing him more effectively with each scenario. Also, this is a slightly altered Vecna (swapped out some spells) with the Book of Vile Darkness. Personally, I think the spell choses made him more effective, but the Book is a factor as well.
I also encouraged the participants to being a private DM chat to discuss plans and strategies, but I don't think much of that (if any) has happened.
While I think these scenarios show a well-honed and well-played Venca is certainly a match for a party of 20th level PCs, without some tweaks, etc. I think the stat-block Vecna presented originally by WotC is a bit lack-luster otherwise.