Pulse or Tail for defending?

You should probably invest immediately into this defender. It'll get you a solid return in both dust and coins. Water Pulse In my experience is the more annoying defensive moveset to face. Once it appears it spam attacks you with WP. I've often gotten hit launching a solar beam with Exeggutor.

Definitely WP for me. It come more often than AT, often twice in row. When you use Solab Beam you are very often hit by ine WP. In addition, WP Animation is a little shorter than AT one

If I were you i'd go for it, high IV WP Vaporeons are 10x more annoying than any Gyarados will ever be. As a defender, Water Pulse is unique in the sense that it doesn't require much energy and occurs frequently. Even if a move is dodged you still take some damage. The viability of the move scales hard with higher IV and CP.

EDIT: You've probably read about this already but Water Pulse causes terrible lag on older phones. In my opinion that definitely adds more value to the move.

WP

Water pulse is by far better choise. I have trained rural gyms up to lvl 9 with a set-up of 2x 2600 vaporeon with AT and other one with 1x 2700 WP. Prestigers were 90% iv tangelas with power whip and solar beam.

First gym I had no problems since their AT comes with a certain rythm. Mostly ended with 10% and 50% hp left.

Second gym. So many knock outs. I cant even remember how many times I was suprised by barrage of WP.

Invest.
For me WP is easy to dodge only using Db Dragonite, with VW Venusaur I tend to get hit 1-2 times, with Exe even more so.
AT is easier to dodge both with Venusaur and Z/P Exe.

For prestige I manage to dodge both of them with Fury cutter or bug bite parasec, I miss some WP with Tangela or Razor leafs users.

They are both pretty good. I'd say WP is probably more predictible than AT in the sense that I know the next WP is never far away. Because of that, I might get hit more often by AT than WP. Versus WP Vaporeons I favor using Venusaur with PB or Exeggutor with Seed Bomb, as I'd eat too many WPs launching Solar Beam. They both announce pretty early, so nobody should be surprised by either of them.

I have two defending Vaporeons 2800cp+ and they have AT and HP. I've found HP annoying because it seems to charge faster than you'd think and it can take you by surprise just when you thought it was safe to launch a charge move.

AT is difficult to dodge, but WP comes so quickly that if the attacker dodges every attacks, the attacker will does less damage to the defender, your Vaporeon may even "time out" the attacker if the attacker eventually dodges WG, and has a quite low base ATK.

Power up the WP one is definitely good, and its high IV makes it valuable.

WP is slightly more annoying on defense.

HOWEVER, if you're short on dust, and you're choosing between the 2 w/ identical Vapes, I'd go Aqua Tail.

Why? B/c it's nice when you can power up a Pokemon that's useful as both a defender AND an attacker, for when they're in between gyms and you need a helping hand.

WP is great on defense but absolutely atrocious on offense. It's so bad, in fact, that it's faster to completely ignore it and just spam Water Gun.

Examples: Vape w/ AT/HP, Dragonite w/ DB, Gyarados w/ HP, Rhydon w/ SE/EQ, etc.

By powering them up, you have a Pokemon that can fill 2 roles.

As per the other post on the front page at the moment, defensive moveset only matters a little bit. It actually hurts your own team more making it harder to prestige IMO