I might be the exception, but I don't have designated "attacking" and "defending" pokemon - I just power up the best overall pokemon I have (e.g., Dragonite, Lapras, Exeggutor, Vaporeon) and use those for everything. While I'll put any of them in gyms, usually I stick in a 2400 CP Vaporeon because while I have 1 Dragonite, 1 Lapras, and 1 Exeggutor, I have 6 excellent Vaporeons.
Long story short, if I were you, and I didn't already have high level pokemon, I'd power up the Lapras and Exeggutor, since they're excellent both for attacking and defending. I would probably ignore the Slowbro unless you have lots of dust/candies, and I would definitely ignore the Vaporeon, since you can easily catch many higher level Eevees in the wild.