1) Vecna is not missing his whole left arm!
2) was there critical hit rules in the 80's ? I always thought it was more of a houserule thing that became a rule in somewhat recent editions.
There weren't officially crit rules, no, but it was a very common house rule. And RAW wasn't really a thing back then--even folks who thought they were playing strictly by the book usually weren't.
In general, any time we're talking about a game from the TSR era and especially 1E, one should assume a heaping dose of a) house rules and b) misunderstandings of the official rules, which were labyrinthine enough to baffle Baphomet himself. In the D&D game that opens the Season 1 premiere, Will has to make an attack roll with a
fireball, which is certainly not by the book... but the kids might have assumed that a damaging spell constituted an attack and thus required an attack roll.
IMO, the only thing that's really
inaccurate is the use of "rogue" instead of "thief." (And I'll be honest, I didn't even notice that until y'all pointed it out.) I'm just going to say that Erica decided on her own to call it "rogue," and then later in life browbeat some D&D designers into adopting that term for later editions.
To the original question--that scene was awesome, and I really appreciated the goodies they tossed in for D&D players. Obviously there's the "Vecna lives!" reference, but I especially loved the reveal where everyone freaks out at the cultist missing a hand and eye. That kind of reveal, getting that kind of reaction, is the sweetest, purest joy a DM can know. (And it was a special gift to D&D fans, because no one else in the audience would have had a clue what it was about.)