One 2000cp or two 1500 Vaperons?

I would choose to have one strong Vaporeon for defense purposes. You may want to have two attacking Vaporeons, but having the risk they both come with Water Pulse is not worth the risk to me.

Given that Eevees aren;t that hard to come by (especially with the halloween candy bonus right now) why not have all 3?

2x 1500cp with hydro pump and a good HP IV are a great tool for levelling up a friendly gym (assuming fire type defenders with greater CP) and still useful for taking down an enemy gym (with dodging you should easily defeat 2200+ arcanine). But for leaving one behind to defend once you have levelled up, a 2000cp+ with hydropump or waterpulse is the way to go - gives you a better chance of getting a higher slot in the gym and collecting rewards over multiple days. The more CP the better!

I'm going to assume this isn't hypothetical, which means there's only 1 possibility unless I'm missing something:
You currently have a 1500CP Vaporeon that you didn't use the name trick for. So you can still use the name trick to guarantee a second one on evolve, or you can use those candies to power up the first one.

So my question would be: What is the IV and moveset of the one you have? I have a 93% IV Vaporeon with Aqua Tail who has been a wonder on both attack and defense. Was definitely worth powering up (currently 2116 with room to grow since I recently leveled up). I also finally evolved my second Vapo last night (so many Flareon and Jolteon) which came out over 1900 CP.

So what I'm saying is - one vapo does fine. Especially if you have Golduck/Starmie/Slowbro./Blastoise to attack with while she defends. If it's 80%+ IV and has the moveset you want (WP for defense, HP for attack, AT for if you want a mix of both) then power it up! If not, just sit on it and wait to catch a stronger Eevee - you'll get one eventually. Much better to outright evolve a 2000CP Vapo and then have 2000CP + 1500CP - the best of both worlds.

Ideally? I'd say as many Vaporeons as possible. I have a trio of 2400+ CP Vaporeons I use for getting my daily gold. Given their CP, they're pretty resistant to getting knocked out of gyms. (At this point, I don't even care about their movesets.)

In the short term, I would probably go for one 2000 CP Vaporeon. I don't know how often you train friendly gyms, but IMO it's more useful to have a small collection of strong, really high CP mons for attacking/defending as opposed to a diverse set of ~1500 CP mons for training. Then again, I almost never train friendly gyms; your play style might be different.

two 1500 is useless. Go power one to 2400

one is better