super effective!Vaporeon is one of the most popular Water type Pokémon, and for good reason. Its massive HP stat and excellent Special Attack make it one of the best Water types to use, in my opinion! I usually set it up with a moveset of Surf, Ice Beam, Aqua Ring/Wish, and Acid Armor/Baton Pass. Surf is a great Water attack with a power of 95 and and 100% accuracy, which means Fire, Ground, and Rock types are easy KOs. Ice Beam provides an attack that can hit Grass, Flying, and Dragon types pretty hard.
Aqua Ring is a great move that offers healing each turn, and it can be passed to other Pokémon via Baton Pass. You'll have to raise an Eevee to level 36 before evolving it to get a Vaporeon with Baton Pass, however. Wish is another excellent healing move, though the method of getting a Vaporeon that knows it is a bit complicated. You need a Togetic or a Togekiss that knows Wish, first off. Togetic can breed with Pikachu or Raichu to get an offspring that knows Wish, or alternately you can breed a Skitty or Delcatty with either Togetic or Togekiss to get an offspring that also knows Wish. Breeding that Pikachu, Raichu, Skitty, Delcatty which knows Wish with an Eevee results in an Eevee that knows Wish! It's a pain to do, honestly, but it can be worth it.
Vaporeon's one downfall is its relatively low Defense stat and its weakness to Grass and Electric moves. Acid Armor does provide a boost in Defense by two levels each use and Ice Beam can effectively take down Grass types, but Vaporeon is best off not being used against Electric types at all. But it does take only 1/2 damage from Fire, Ice, and Steel attacks, plus it completely absorbs Water type attacks thanks to its ability, which heals 1/4 of its HP when hit by a Water attack. Awesome. Its Dream World ability, Hydration, allows Vaporeon to recover from status ailments while it's raining. I generally prefer Water Absorb, but if you're running a Rain Dance team or something of the sort, Hydration might be a better choice. If you're into EVs and breeding and natures and all that jazz, check out Smogon's page on Vaporeon.