Up until today, evolving your Eevee on Pokémon Go was seen as a random event, you didn’t get to choose exactly which evolution your Eevee would become, it was just a matter of chance. Now, there’s a way for you to choose exactly what evolution you want.
The way for you to do this is pretty simple, though for someone like me who likes to keep their Pokémon’s original name, it means you’ll have to compromise. In short, in order to guarantee your Eevee’s evolution is the one you want, you’ll have to rename it either Rainer, Pyro, or Sparky.
In a post over on reddit, some users have discovered a way for you to choose whether your Eevee evolves into Jolteon, Vaporeon, or Flareon. The way this works is by renaming your Eevee to one of the three names of the Eevee Brothers from the original anime.
It would seem that there is in fact a way to force an evolution of your Eevee. The Eevee Brothers from the original anime were named Rainer, Pyro, Sparky and they owned a Vaporeon, Flareon and Jolteon respectively. If you nickname your Eevee either Rainer, Pyro or Sparky, your Eevee will evolve into that desired Eeveelution!
So, if you want a Vaporeon, you’ll need to rename your Eevee to Rainer, for Flareon it’s Pyro, and for Jolteon, it’s Sparky. The reddit user tried this on three different occasions with each name and ended up with their respective Eeveelutions.
Due to the stability of the servers, the reddit user did advise to restart the game once you’ve renamed your Eevee just to confirm that the changes have happened on the server.
Then evolve away!