Two days to go! Been re-watching the show from the beginning over the last few weeks, and found a possible Vecna reference back in S1 E6. When El approaches the Demogorgon in the “Void” as it’s feeding, you can hear a clock ticking. Doesn’t necessarily mean anything, but there’s no real reason for it to be there otherwise.
Apparently there’s also clock chimes on a couple of occasions, including when Will crashes his bike in the first episode?