Like Chansey, bulk is the only thing it had going for it. Water gun is absolutely horrible on defense. The only annoying move was aqua tail, that would spam like BS Snorlax. You also chose an ideal counter, one with good bulk and resistance to water. Back in the prestiging days I could get through a Vaporeon and part of another with a level 20 Tangela. Blissey was really the only pokemon that was difficult with lower level pokemon. If you play around, you will find a lot of pokemon perform better than one would expect, such as Pidegot and Noctowl. My favorite Eggy slayer was butterfree.
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I finally knew why Vaporeon doesn't deserve a spot in the main defender tier list
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I still keep some Vaporeon though, cannot get rid of them for memories sake, it is a beast in Gym Meta in Gen 1.
Has a bulk and Water Gun is quite good at that time
I have a squad of 6 Vaporeon at lv 35-38 ready just to break down the gym when i just feel lazy to pick a good counter
And i am not attacking with dragonite, gyarados, or rhydon, all go to the gym defender role along with snorlax..
It become a sad day when the pink monster blissey come, as the water gun king cannot beat a high level blissey cos they nerfed water gun.