Before last year, their policy had been to only release 1 skin per champion per year, but here we have Jhin who got 2 skins on the regular production run (Blood Moon, then Project) and then Vi who got the same (Warring Kingdoms, then Project). Riot has been throwing their past policy to the wind, so it's now extremely hard to predict what they'll do.
As a note, because I'm sure someone will mention this - SKT T1 Jhin was not on the regular production cycle last year. Worlds skins are special circumstances because they are chosen by the winners, and don't move through the same processes that regular skin concepting and design goes though.