The OP is referring to https://pokemongo.gamepress.gg/houndoom-duo-raid-guide (which omits Gyarados), not https://pokemongo.gamepress.gg/raid-boss-counter/houndoom-raid-counter-guide (where Gyarados is listed).
Gyarados has less bulk than Vaporeon and is weak to Rock-type attacks.
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But Houndoom doesn't have rock-type attacks. I would think that for those without Kyogre, a good Waterfall/HP Gyarados should be an essential part of the team (unless you're swimming in Machamp).
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We write the guides in Google Docs and sometimes when they are ported to the site, new errors get introduced. This is now fixed.
Vaporeon definitely is viable in rain, I don't know what you are talking about.
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Oh, my bad. That was dodging all charged moves. Without dodging, Vaporeons have 60% chance to win.