If something happens "when an attack is made", when does that something happen?

Question

A select few features occur, not when an attack roll is made, but instead when an attack is made.

Three examples are the Protection Fighting Style, the Arrow-Catching Shield magic item, and the Vengeance Paladin's Soul of Vengeance feature. These state (emphasis mine):

When a creature you can see attacks a target other than you that is within 5 feet of you, you can use your reaction to impose disadvantage on the attack roll. [...]

[...] In addition, whenever an attacker makes a ranged attack against a target within 5 feet of you, you can use your reaction to become the target of the attack instead. [...]

[...] When a creature under the effect of your Vow of Enmity makes an attack, you can use your reaction to make a melee weapon attack against that creature if it is within range. [...]

Notably, there are numerous other features that occur when somebody is targeted by an attack, or when somebody makes an attack roll, or when somebody is actually hit with an attack, but none of the features above specify any of those scenarios. Examples of features that do specify are the Mastermind Rogue's Misdirection feature, the Lore Bard's Cutting Words feature, and the Monk's Deflect Missiles feature:

[...] When you are targeted by an attack [...]

[...] When a creature that you can see within 60 feet of you makes an attack roll [...]

[...] when you are hit by a ranged weapon attack. [...]

As such, I'm unsure when the Protection Fighting Style, Soul of Vengeance, and Arrow-Catching Shield actually takes place. Are they before the target is determined? Before the attack roll is made? After the attack roll but before knowing if it hits or misses? After knowing if it hits or misses? After the damage is applied? Does it vary with each feature? Is it some other answer entirely?

Answer 1

When it makes sense.

This is a little hard to talk about because for many of these there is no defined "when" within the fiction. For example, the Protection fighting style move represents positioning yourself somehow to make the target harder to hit. There's no specific moment at which you do that; it's just what you're doing while fighting next to them.

If you mean "when" in terms of the order of operations at the table, though, you do it when it's appropriate to make the mechanic work. Usually this is obvious from what it does.

Taking these in order:

[Protection fighter] When a creature you can see attacks a target other than you that is within 5 feet of you, you can use your reaction to impose disadvantage on the attack roll.

There's only one possible "when" here. It only applies if the target is within 5 feet of you, so you must do it after the attacker announces their target. But it modifies the way the attack is rolled, so you have to declare it before the attack is rolled. So between the attacker announcing the target and rolling the attack, you have to jump in and say you're using this.

[Arrow-Catching Shield] whenever an attacker makes a ranged attack against a target within 5 feet of you, you can use your reaction to become the target of the attack instead.

This is a little more slippery, but for consistency with the rest of how making an attack works, it should happen at the time the target is announced. They announce they're shooting at Bob, and Alice says "No, you're shooting at me", and the attack proceeds with Alice as the target.

The reason to handle it this way is to prevent intervening events that depend on who the target is. From the perspective of the other mechanics, the attack only ever had one target.

[Oath of Vengeance paladin] When a creature under the effect of your Vow of Enmity makes an attack, you can use your reaction to make a melee weapon attack against that creature if it is within range.

This doesn't modify the attack in any way, so it can be handled under the general rule in the DMG: "The reaction occurs after its trigger finishes, as in the Ready action." In this case that's the entire process of making the attack: roll the attack and apply damage, then the paladin gets to make their counterattack.

Answer 2

"Making an attack" and "making an attack roll" are the same thing.

The rules do not seem to distinguish between "making an attack" and "making an attack roll". This is spelled out in the rules for "Making an Attack". First, the introductory sentence says:

Whether you're striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure.

Here, we see a sentence structure that seems to be equating "striking with a melee weapon", "firing a weapon at range", and "making an attack roll as part of a spell". This identification is made explicit a few sentences later:

if you're making an attack roll, you're making an attack.

"Making an attack" and "making an attack roll" are the same thing.

Except when making a grapple or shove

It is less clear when the attack made is a grapple or shove, but it seems entirely natural to follow the model of "rolling the dice is making an attack", and rule that an attack is made at the time the associated ability check is rolled in the case of grappling and shoving.