In the original game, evolution was dictated by elemental stone you provided. In Pokemon GO its seems to be quite the Easter Egg.
In the original show, there were 3 Eevee trainers. Rainer, Sparky, and Pyro.
Now if you actually name your pokemon one of these names, he will evolve into the corresponding element!
NEATO RIGHT?
Sparky will get you a Jolteon
Rainer will get you a Vaporeon
Pyro will get you a Flareon
Now if this doens't work, the other rumor surrounds the move that your Eevee knows as his special.
Flareon comes from an eevee that knows Dig
Vaporeon comes from an eevee that knows Swift
Jolteon comes from an eevee that knows Body Slam