Kingdra vs Vaporeon

Dragon gives it extra fire resistance, so it would be excellent for fire Pokemon. I think Exeggutor with the right moveset is still better for Vaporeon, but it certainly beats using Dragonite versus Vaporeon imo. Better than Jolteon. So rankwise your group vs Vape is...
1. Exeggutor (if SoB)
2. Kingdra
3. Dragonite
4. Jolteon.

Mindrivet is spot on. So I guess it comes down to gameplay. Do you dodge, do revive/potions count, stardust, perhaps you counter gyms more than prestigie?

91 is decent. My little1970/73IV comes in handy against Vapes and gyms full of Dragonites when potions are short.

I have a decent Kingdra but it's easier and more fun to use Grass. As a prestiger it might shine, never tried, too high level.

Ironically the only thing Kingdra is good for is fighting Vaporeon. So if you need to beat Vaporeon then level up Kingdra, otherwise level up Vaporeon.

I regard DB/O-Kingdra as universal prestiger. It is perfect, when Fire and Water defenders stack up directly above each other - no urge to change attacker then, Kingdra is double resistant to both.

If you want to fight waters, i consider tangela better, if you want to fight fires, i consider vaporeon (or double-water-kingdra) better