Choose a Vaporeon

If it was me I would max the first one for sure, 100%IV mons with good moveset and usable are a must to power. I won't max a 100%IV Raticate with B/HB for example, just use him as prestiger, but a Vaporeon?! Without even thinking it twice for sure.

Eevees are sooooo common, I've stopped all powering up of the evolutions. I'll evolve the high IV ones but then leave them where they are at in order to use stardust on other mons.
If you need a high cp Vap to drop in a gym then the least stardust expensive one is #3.
No reason spend all that dust on #1 or #2, but you can bring them up to 1200 to be your goto prestige mons.

Power up 1 and 2 to 1000~1200 CP to use for training.

Power up 3 a few times to level 30 to use for a backup attacker against Rhydon heavy gyms and a pinch defender when you run out of other options. Each time you place it in a gym, you can consider powering it up a little more to place higher.

3, 1, 2 - in that order. 1-2 IV pts don't make that much of a difference.

If you'd rather, 3 is already viable as a gym defender in less active regions, so I'd start with 2 then.

IMO do 3 first.

Vaporeon prestiger is okay, but you're better off with other pokes since the majority of vaporeons cp is take from its massive stamina stat.

Honestly I'd probably only power up #3, assuming you'll use it as a defender (if you're just looking for attackers then you could power up to L30 if you really want to, though it won't make a huge difference). Otherwise, I wouldn't power up any of them - just leave them where they are as prestigers. Eevees are so common that it's a better option just to evolve the high-level ones and save your dust.

Usually I'd say ignore IVs and power up the current highest CP mon (to save resources), but 100% IVs are an exception. Assuming you're like me and have lots and lots of dust and nothing to use it on, go for number #1. This has the added benefit that you'll end up with two high CP Vaporeons. If you want to be more conservative with your resources, stick with #3.

Number 1 will max out over 3100 CP at Pokémon level 39, and AT is a very versatile moveset for Vape. Great attacker and defender, so number 1 for me if you've got a bunch of dust.

Spending 100k+ dust and 100+ candy to take a Vape from lvl 10 to lvl 28 for 2 ATK IV? AKA 23 CP?

No thx, I'd rather have a 96% Vape + 100k dust + 100 candy than a 100% Vape and 0 dust / 0 candy. You can have your 23 CP. I'll just evolve 4 more Eevees instead and stock my bench for gym battling marathons.

But I took the road less traveled from the beginning. I only really power up defenders, where every CP matters. But even in this case, 100k dust for 23 CP ain't worth it IMO - esp for Vape where he's a few hundred CP away from Lapras/Gyarados anyway. It's only useful for pushing him over other Vapes.

Now I have a ton of strong attackers at lvl 30-31 instead of a handful at lvl 36-37. It's come in handy on many occasions. Nobody needs to know they're only 70-80% IV's. YOLO

Obviously you want to power up the 3rd one, 13 on Attack isn't much of a loss from 15, and 2503 is way higher than 902

I wouldn't touch #2 or #1, but I would max #3. I see no reason as to why you would spend a ton of dust on #1 with minimal difference from #3. I'm pretty jealous you have a 100% WP.

I'd go 1 and then 3, the candies are no issue and dust is dust... Less you are super short and have better mons you wanna power up.

That added little safe feeling that your Vaporeon will sit higher than almost any other of a player at your level would make it worth it for me. While 9 times out of 10 it doesn't really matter since most people have already concluded that a gym is going to the ground when they decided to attack it, but it always feels nice to sit up a little higher.

Pulse is considered better, but a Vap with tail is a better defender than about 75 % of things people put in gyms as defenders. Plus, if they aren't in gyms they are also super fast and effecient attackers. Unless the 130-150k of dust is a deal breaker, which it wouldn't be more me, and I'm always low (140k right now) I would no doubt power that bad boy up!

If you have more than 500k dust 3 than 1
If you are short of dust only 3