It depends how early in the creative process you silo off "vision" vs. "implementation". If the vision is purely "history built in layers", then Age resets aren't even necessarily a part of it. That's implementation.From what we can reconstruct, I don't think mixing civilizations and leaders was part of the vision. We know that Firaxis playtested other options, like keeping civilization through ages with changing leaders. So, I'd say that original vision included age reset only and things like changeable civilizations and mixing leaders with them, just came naturally from the vision.
This also fits with usual definition of vision, where it's something really short and usually aligned with marketing message. So it probably was something around "History built in layers".
I'm being a bit more permissive than you, so I include fundamental attempts at implementing the vision as a part of the overall goal for the product (pre-release). Certainly, by the time we were getting dev blogs on each of the major features.