According to [this post on the /r/pokemongo subreddit][1], you can force your Eevee to evolve a certain way by changing its nickname before evolving it to:

* *Pyro* to evolve into **Flareon**.
* *Sparky* to evolve into **Jolteon**.
* *Rainer* to evolve into **Vaporeon**.

(These are the names of [Eevee Brothers][2] from the TV series/Manga)

You can also, as of the Generation II update, use nicknames to evolve into a certain Eeveelution, but like before, this works only once.

* *Tamao* to evolve into **Umbreon**.
* *Sakura* to evolve into **Espeon**.

If your Eevee has any other nickname, the evolution will be completely random.

This technique will only work **once** per evolution. For example, if you have already evolved an Eevee named *Rainer* into a Vaporeon, evolving another Eevee named *Rainer* will result in a random evolution.


  [1]: https://www.reddit.com/r/pokemongo/comments/4svayv/how_to_choose_your_eeveelution/
  [2]: http://bulbapedia.bulbagarden.net/wiki/Eevee_brothers