Catch Eevee in the wild
Use 25 Eevee Candy to evolve Eevee
Evolve Eevee during the day without using a nickname if you want a random evolution, which can be Vaporeon
Use the nickname “Rainer” once per account to guarantee Vaporeon
Find Vaporeon in raids or special event encounters if it is featured
Trade for a Vaporeon from another player
Hatch Eevee from eggs, then evolve it with 25 Eevee Candy
Complete research or event tasks that reward Eevee, then evolve it into Vaporeon