Catch an Eevee
Collect 25 Eevee Candy
Evolve Eevee
Use the nickname trick once by renaming Eevee to Rainer before evolving
Use a Water-type evolution item only if a special event or game mechanic requires it
Walk Eevee as your buddy if needed for a specific evolution task
Check for event bonuses that may affect Eevee evolutions
Evolve Eevee during normal gameplay to get Vaporeon randomly if the nickname trick has already been used