To name an Eevee in order to evolve it into a Vaporeon, you can use the name "Rainer". This is part of a naming trick that only works once per evolution, so its important to plan ahead before using it. Other names that can be used to evolve Eevee into different forms include "Sparky" for Jolteon, "Pyro" for Flareon, "Sakura" for Espeon, and "Tamao" for Umbreon.