@ MonsterEnvy...I know this will really bog down the simulation, but maybe run it as play by post, where each player runs the char?
But to be fair, I am also happy you running my char.
A couple of comments on my char Arthur:
1. How many Javelin's would he be carrying, given he is in full plate, plus shield, plus long sword.
2. This gets very meta-gamey (which I hate). So a few things:
a. Arthur has no idea of the abilities of Vecna, so will attack with Long Sword until Vecna teleports away after the 1st of potentially two long sword attacks. At that point Arthur will consider using Javelins or spells, but also knows that a Divine Smite with a 5th level spell slot is a huge deal.
b. Does Arthur learn after a round or two, or three, quantities of Reactions and LR's that Vecna has and plan attacks accordingly?
c. Arthur will most definitely close to within 10 feet to trigger the bonus action Channel Divinity Vow of enmity to gain Advantage on attacks. That is likely not the most optimal group move for the 1st turn, but given Arthur does not know the full power of Vecna, he would do that.
Re: Metagaming, a DM has to be very careful, and very tough, with players who will already know this stat block and plan accordingly. I am talking about in a real D&D session.