I’ve been rewatching the show from the beginning over the last 2-3 weeks, as a refresher ahead of Volume 2 this Friday, and came across a couple of moments in S1 that I thought could suggest the Duffers have had Vecna planned out well in advance.
Most notable one is in E6 (The Monster), when El has her flashback to when she made contact with the Demogorgon in the Void/mind space. During that whole scene, you can hear a clock ticking in the background. Obviously could be completely unrelated, though there’s no real reason for it to be there otherwise.
Also, back in the first episode, there’s what could be construed as a clock chiming twice when Will crashes his bike.