100% IV Eevee

Espeon! Everyone has vaporeon and its a good change with about equal power.

I would go with Vape for two reasons:

First, it's still a very important attacker and defender in the current meta. Second, you have better odds of getting a viable moveset, 2/3 chance of getting something good.

I would pick Vaporeon. It can pick apart Tyranitars and Rhydons.

Further, water gun is fun to use.

Vaporeon

I have a L1 100% IV Eevee and am probably going to make him an Espeon. But if I had the option for a guaranteed Vape I would probably go for it for the same reasons pitmound listed.

In all Eevee's evolved form Vaporeon is the best of them...

Believe it or not, I'll choose Flareon or Espeon, but not Vaporeon.

Male or female?

Prejudice aside, Vaporeon makes the most sense for reasons given above.

It will come to the moveset you get in the end. In case you are unlucky, Vaporeon's worst moveset will always be slightly more valuable than Espeon's worst moveset. In terms of gym battling. Saying that I would still evolve the one I like the most and take the risk.
Espeon 100% being able to reach over 2900 CP would be my choice, because vaporeon can easily reach over 2900 CP even not being perfect.