Heads Up Guys!
An eevee theory has finally been confirmed(?)
Before evolving your eevee you have to nickname it one of the 3 eevee bros from the original anime
Pyro
This will get you flareon
Sparky
This will get you jolteon
Rainer
This will get you vaporeon
Keep in mind there's still a 33% chance of getting a random eeveelution
From the original reddit post itself
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Edit 6: /u/datapawwolf suggested something worth noting. With all of the server issues we constantly deal with, it is recommended to name your Eevee with your intended evolution name, then close and restart the app to confirm the name went through on the servers. Some users reporting it didn't work and we believe this may be the reason why.
Edit 6: It would seem a lot of people are reporting that their first Eevee evolved into the correct evolution, but the second Eevee may not have. I think it's possible you may only be able to obtain one specific Evolution at a time, and not obtain the same one twice in a row. Please continue to post results so we can try to get some real data!"