Wanting that specific evolution for your highest CP Eevee? Well a fun easter egg was discovered in the game, if you name your Eevee after one of the original trainers from the Eevee episode of the original Pokemon Cartoon, you’ll get that specific one. I’ve done it multiple times myself and can confirm it works. There can be a delay on the server saving the name so make sure that you follow the steps to get your specific Eevee evolution.
Name your Eevee appropriately for which evolution you want
Vaporeon = Rainer
Jolteon = Sparky
Flareon = Pyro
After you’ve named your Eevee, it’s time for the next step!
Make sure you’ve named your Eevee, then close out Pokemon Go, and then log back in. Your Eevee should have the name of what you changed it to, if it doesn’t rename it and repeat this step. Once you log in and it’s name is changed it’s time to evolve! After you evolve them, they will be the correct evolution!
Hope that helps everyone wanting that specific evolution!