>>58165278That was Motofumi Fujiwara. He was the guy who designed Eevee, along with Flareon and Jolteon. Nishida then used Fujiwara's design philosophy to create Vaporeon and the rest of the Eeveelutions.
It's worth noting that Sugimori also had some input in their design; he revised Vaporeon into what it is today.
Source:
https://lavacutcontent.com/ken-sugimori-eevee-origins/