Others could also have run stat blocks (the way they used to be) in any fashion they wanted, as well, but WotC felt such changes were necessary.
So, instead of people converting the old stat blocks to get they want, now I (and others) have to convert the new format???
Vecna has the spellcasting trait, so should abide by those rules. A game that insists on different rules/systems of monsters and another for PCs is not well designed IMO. Vecna could have kept his spells as was formerly done, and given him other features if DMs desired.
And creates OTHER
complaints in the meanwhile.
Why does this only apply to casters?
After all, the non-caster fighter types certainly don't follow PC rules. What class is a Knight? What class grants me this non magical special ability:
Leadership (Recharges after a Short or Long Rest)
: For 1 minute, the knight can utter a Special Command
or warning whenever a nonhostile creature that it can see within 30 ft. of it makes an Attack
roll or a saving throw. The creature can add a d4 to its roll provided it can hear and understand the knight. A creature can benefit from only one Leadership die at a time. This Effect
ends if the knight is Incapacitated
So, why aren't people all up in arms about how the non-casters don't follow class rules?
As far as "not being well designed", well, that's 5e for you. It has been this way since day 1. Monsters have NEVER followed PC rules. That's a 3e thing that never made it out of 3e. It certainly was never true in 5e. So, since you haven't been banging the "D&D is poorly designed" drum until now, what's the actual problem? Or should we go back to the days when orcs only had warrior class and had to be leveled up to advance them?
So, no, you're flat out wrong here. It's not poor design. It's the way the game has always been designed. They went with spell slots as per class previously, but, well, that turned out to be a huge waste of time and space. Why bother giving Vecna, what, something like 40 spells known, when he's going to cast, maybe 5 of them? The days of monsters following PC rules ended fifteen years ago and hasn't been true since the release of 4e and was only true for the 3e and 3.5 e's.