Vaporeon worth investing in?

A Hydro Pump one is also good for gym defense, because of the dodge bug.

How much stardust do you have? I personally would never power them up, wait for a better Iv eevee with hopefully high cp to start with. It's possible to find eevees that would go up to like 2500 without powering up and save so much stardust. Also with solar beam buff vape isn't that good a defender.

I would say to not power either of them, and find a high level one in the wild and evolve that instead. Once you're high 20s, up to 30, a wild Eevee that's around 750 CP, if it evolves into Vaporeon will have about 2200 CP, and it'll only cost you 25 Eevee candies.

If they have low IVs then there isn't any benefit to powering up these over evolving a higher level one which would cost you less resources. If they had really high IVs and a good moveset then you could be justified in spending the extra dust/candies - since you're essentially spending more dust/candies for higher max potential.

Still keep lower level ones though when you do get higher CP Eeveelutions in the future, Vaporeons are really good for prestiging against Rhydon, especially with Aqua Tail, which is fast enough to be used in between quick moves and still dodge.

FYI. I evolved a 770CP eevee into a 2603 vape. 0 stardust expense.

I personally wouldn't invest in one that's starting from the 1600s with the intention of it being a defender, especially if doesn't have stellar IV. If you want them to be attackers, sure. AT is a good move on offense.

Defenitely not, eevee isn't rare at all you should be more patient and wait for better IV eevee, I know you might get flareon or jolteon instead of vape, but with those IVs, your vapes don't deserve your precious stardust. Remember collecting stardust is much much harder than eevee candies. Even when you get a good IV vape with AT, you should only power it up if its lvl is already high, like 28++ to save stardust and only if you desperately need a gym defender since snorlax, blissey, and dragonite are obviously better defenders and some other high CP mons might be better defender too. The problem with a defender vape is not the AT, but the WG, based on my experience, I agree with you that AT is a great defending charged move but WG is terrible at defending, it barely deals damage. I'm not saying that AT vapes are bad defenders, it's just there are better options unless you desperately need a new defender.

Too low starting level plus too low IVs to be worth considering investing that much dust. For me, the lower the starting level, the higher the IVs are going to need to be to justify investing stardust. At that level, IVs would need to be near perfect (preferably with 15 attack) for the most CP gain.

Save them for prestige against really high Rhydons and Tyranitars.