Lack of STAB hurts here. Scald or Surf needs to be somewhere on there.
And I just found out something bulky that can help set up Rain Dance over Vaporeon: Slowbro. While it doesn't hit as hard as Vaporeon does, it has access to reliable recovery in Slack Off and Regenerator:
-Rain Dance
-Slack Off
-Scald/Surf
-Psyshock
Nature: Bold
EVs: 248 HP/244 Def/16 SAtk
Item: Leftovers
Ability: Regenerator
(you don't have to use this if you don't want to)
Gyarados isn't
that frail; its HP is decent and Intimidate compensates for its lower Defense in comparison to its high Special Defense. It has the same bulk as Milotic, to put into perspective.