So i was about to evolve my two highest cp eevee hoping for some good cp. My 255 cp eevee evolved into a vaporeon with 700 cp . And my 205 cp eevee evolved into a 400 cp jolteon. Can someone explain how this works so i know for future evolutions?
Each Pokemon including the three Eeveelutions have a different min and max CP. You will notice an arc behind the Pokemon when you select it. This arc represents the percentage of the Pokemon's max CP. So if the arc is half way and the Pokemon is 500 CP if you power it up to max, it will get to 1000 CP. The max CP also increases with your trainer level pushing the filled portion of the arc back a little with each level up.
When you evolve a Pokemon the percentage of the arc will remain the same, but due to the max of the Pokemon increasing, the CP will increase when you evolve.
The reason your Vaporeon gained so much more than your Jolteon is because Vaporeon has a higher max CP than Jolteon.
However note that Jolteon has a really fast attack speed, which it seems isn't accounted for in the CP calculation which is why it is so much lower than the other two eeveelutions. But from my experience the fast attack speed makes it far stronger than slightly higher CP Vaporeons and Flareons.