Vaporeon was a mediocre defensive Pokemon outclassed by Suicune for the majority of GSC's history. Its draws over Suicune were access to Reflect and Haze, but ultimately not having that monstrous Defense stat left it unused in OU. Then, the NYPC gods gave Vaporeon and its Eeveelution brethren a gift from Pokemon heaven—Growth—which catapulted Vaporeon into legitimate OU territory. Growth is the only move in GSC that boosts the Special Attack stat, and Vaporeon is by far the best user for a standalone special sweep thanks to its bulk and high Special Attack. Nowadays, while Vaporeon still has the same defensive prowess, Growth boosts its Special Attack, which is high even without a boost but often overlooked, and makes it one of the premier offensive threats in the game. Vaporeon is a threat that must not be underestimated despite its initial lack of popularity.
Vaporeon is a unique offensive Pokemon in GSC OU. It's a legitimate special threat that can take a hit and can set up with Growth. Unlike the legendary Electric-types that are GSC's other prominent special threats, Vaporeon is more geared towards setting up and sweeping rather than scoring opportunistic KOs. Vaporeon sets up on the special side, which makes it immune to stall tactics such as Growl and Charm, as opposed to other reliable setup Pokemon such as Curse Snorlax. Dissimilar to more daring setup Pokemon such as Belly Drum Snorlax and Swords Dance Marowak, Vaporeon is able to break stall while having an easy-to-use setup move and the ability to take hits.
Vaporeon's STAB is the choice between reliability and power. Hydro Pump is great for wallbreaking. It can 3HKO Snorlax after a single Growth and it also does 36-43% to Raikou without a boost, meaning that if Spikes are down and Vaporeon's Hydro Pump connects, Raikou can only afford to switch in once more before having to use Rest. Surf, on the other hand, isn't going to miss and doesn't suffer from Hydro Pump's horrendous 8 PP. Growth is the whole point of the set, allowing Vaporeon to take advantage of its good defenses and set up the sweep. Rest keeps it alive and gives it the longevity that allows it to play defensively and to set up when needed. As for the fourth move, Sleep Talk is the standard and keeps Vaporeon around to tank hits from Ground- and Rock-type Pokemon. Furthermore, Sleep Talk also minimizes downtime when trying to set up. Acid Armor, however, beats CurseLax in a head-to-head match-up. Acid Armor is also nice for PP wars with its incredible 61 PP, and buffers Vaporeon against other dangerous physical attackers such as Heracross, Machamp, and Explosion users.Team Options & Additional Comments >>>
As for options in the fourth moveslot of the main set, Vaporeon has a couple of okay options other than the ones listed. Hidden Power Electric 3HKOes Suicune as well as other Vaporeon at +1 and also wrecks the odd Tentacruel looking for a free boost. Blizzard is also an option as it can 2HKO Zapdos without a boost and OHKOes Exeggutor about 50% of the time at +1 after factoring accuracy. However, Blizzard's low PP and accuracy are major turnoffs.
A Baton Pass set could work. In this case, Vaporeon is best off forgoing Rest for Acid Armor as a second stat boost to pass. Good recipients include Zapdos, Raikou, and Exeggutor. Growth boosts the recipient's Special Attack to scary levels, while Acid Armor boosts Defense to render the normally threatening Snorlax unable to do meaningful damage in return. Vaporeon is a great user of Baton Pass because, aside from Roar Suicune, no other popular phazer is capable of stopping Vaporeon from passing its boosts without taking a lot of damage from a boosted Hydro Pump or Surf. Even then, Suicune can be beaten by using Roar on Baton Pass Vaporeon. However, Raikou will usually make life difficult for a Baton Pass set. While it can't comfortably use Roar against Vaporeon without taking a lot of damage in return, it will do tons of damage to Vaporeon if it decides to stay in against Raikou, and it can tank most special attacking Baton Pass targets, even at +1. Zapdos is also a tough foe for Baton Pass Vaporeon; it cannot tank +1 special attackers the way Raikou can, but it does heavy damage to Vaporeon if it decides to stay in to use its slower Baton Pass, thereby preventing Vaporeon from trying again later in the match.
The classic pre-Growth defensive Vaporeon set of Rest, Surf, Haze, and Reflect is an option for those who want a Suicune with worse defenses but better support moves. However, Vaporeon is better off using Growth to play offensively than trying to be a Suicune, especially since it was BL before the advent of Growth.
Checks and Counters
Most of Vaporeon's common checks don't wall it completely, but rather threaten to smash its face in. Zapdos and Raikou are the most common checks and can 2HKO with their STABs. However, they can't take too many Surfs or Hydro Pumps on the switch. Snorlax is the second most common answer to Vaporeon, but it's a shaky check. Snorlax is 3HKOed by a +1 Hydro Pump and Acid Armor variants of Vaporeon beat it without relying on a move with iffy accuracy. Exeggutor is a good stop-gap against Vaporeon but won't last forever. It's easily targeted by Pursuit, dies quickly to Ice Beam Vaporeon, and if Spikes are down it will eventually be put into +1 Surf or Hydro Pump KO range. Cloyster can outspeed Vaporeon and Explode on it, but Cloyster isn't doing anything to stop Vaporeon without sacrificing itself with Explosion. Gengar 3HKOes with an Electric-type attack, and can Explode or use Destiny Bond if things go south, although it struggles to take Vaporeon's attacks. As for less common checks, Tentacruel resists Surf and sets up a counter-sweep with Swords Dance. Dragonite resists Surf and threatens Vaporeon with Thunder and Double-Edge, but loses to Ice Beam variants. Heracross outspeeds Vaporeon and very narrowly misses out on 2HKOing it at full health, although Acid Armor mitigates this damage and Heracross has trouble switching into Vaporeon.
While Vaporeon is usually countered by using high damage output, there are quite a few Pokemon that wall it entirely. Roar Suicune is the most common full stop to Vaporeon, although it fails to stop last-Pokemon Vaporeon or variants that use Roar themselves. Suicune can also play to freeze Vaporeon with both Ice Beam and Mirror Coat, with Mirror Coat keeping Vaporeon wary of attacking while Suicune tries to freeze it. Aside from Suicune, Light Screen Blissey can help teammates such as Zapdos to switch more easily into Vaporeon. It can also try to outstall Vaporeon, but Blissey will usually lose the PP war and, on turns when Light Screen wears off, a combination of a high damage roll and a critical hit from a +5 Surf will put Blissey in KO range on the following turn. Vaporeon can also use Ice Beam to freeze Blissey. At the very least, Light Screen Blissey will be difficult to defeat for Hydro Pump Vaporeon and its meager 8 PP. Starmie resists Surf and can use Thunder hoping for three consecutive hits to 3HKO Vaporeon or Light Screen to flat-out stall Vaporeon out of PP. Starmie, unlike Blissey, can outstall Vaporeon because of Rapid Spin's high PP. As for less common walls to Vaporeon, Meganium can wall Vaporeon with Leech Seed, Synthesis, and possibly Light Screen, although Ice Beam Vaporeon can beat it. In addition, Kingdra can Curse in Vaporeon's face, uses Haze, or even go for the freeze with Ice Beam while only being 5HKOed by +5 Surfs, although it can be 2HKOed by +5 Ice Beams or stalled out by Acid Armor variants.
Furthermore, Vaporeon is a good check to itself. Even after switching in to give the opposing Vaporeon a turn advantage, a Sleep Talking Vaporeon has only slightly less than even odds at beating an opposing Sleep Talk Vaporeon and better odds if the opposing Vaporeon is not a Sleep Talking variant. +4 Surf and +3 Hydro Pump both 3HKO an opposing Vaporeon, so boost this far before you stop setting up and start attacking. Alternatively, you can just use Roar so that you can phaze opposing Vaporeon after you gather a couple of boosts in the Growth war.