I have several higher evees, one of which I am going to evolve. One evee, at cp 267, has a 70 dig attack, and the other cp 325 evee has a much weaker 40 attack. Will evolving the lower evee produce an über powerful attack? I am hoping so, as this will be my first vaporeon, and it will complete my evee team.
When you evolve a Pokemon, their IVs (Attack, Damage, and Stamina) will all stay the exact same as the pre-evolved form, as will their level. As a result, an evolved Pokemon will also experience a CP gain to keep them at the same level. Therefore, it makes more sense to evolve the higher-IV Eevee.
However, there is no guarantee as to what moves you get when you evolve a Pokemon, as they are randomly selected from an "allowable list" from each species. Therefore, your high-IV Eevee may become a high-IV Flareon with a horrible moveset. At the same time, your low-IV Eevee may become a low-IV Jolteon with the best possible moveset.
Typically, it is recommended to evolve the higher IV Pokemon, though.