According to a reddit post by /u/smithnigel, backed up by others and video proof, you can actually make your Eevee evolve into your desired evolution.
How to Choose your Eevee Evolution
Eevee needs 25 Eevee candy to evolve and if you do it normally, you get a random evolution.
If you change the name of your Eevee to one of the following, it will evolve into the eeveelution of your choice.
Vaporeon: Ranier Flareon: Pyro Jolteon: Sparky Espeon: Sakura Umbreon: Tamao
These are the names of the Eevee Brothers in the Pokemon anime and each had a different evolution. Reddit user /u/strongscience62 also showed a video of him naming Eevee Rainer and evolving it into Vaporeon.
Make sure it is spelled correctly and try closing the app and opening it again to be sure the server updated the name change before you evolve it. Let me know in the comments if this worked for you so we can spread the message around!
