What you don't get is that Vettel isn't only a couple of tenths slower than Hamilton, but 1.1 second, which is massive. He did his lap in the dry as did Hamilton almost 5 years after his first try in the wet. Vettel won't find 1.1 second.
I don't know if Vettel would beat it or not, but any top driver _should_ be closer than 1 second.
Learning the circuit may not be that hard but squeezing ultimate performance from a consumer FWD car when you spend pretty much the whole year driving RWD is maybe not that immediate.
I think Hamilton had an itch with this (seems pretty clear from the interview) and practiced to obliterate the lap time. It would not surprise me if he had bought a KIA (peanuts for him) and trained with it in some circuit during the off-season.