But the differences between the Ocampa and Vulcans are too big for them to belong to the vulcanoid species.
I didn't propose that they were the same species, just related species descended from a common ancestor such as Sargon's people. Half a million years is enough time for substantial evolutionary divergence to occur. Even over a shorter timespan, an advanced civilization colonizing multiple planets might employ genetic engineering to adapt different populations to different environments, as the Illyrians have been established to do in Strange New Worlds. That could account for, say, engineering one colony population to have copper-based blood to be compatible with the biochemistry of the planet it settled. (Or engineering a naturally copper-based species to have iron-based blood, but since copper-based is less common, the former premise seems more likely.)