Does the defender have to pay the red power first to choose to get +2 since their ability resolves first, or does the attacker have to pay first when a Draven Vanquisher and an opponent's Draven Vanquisher fight each other?

In a combat between two Draven, Vanquisher units, the Defender's ability resolves first, and therefore the Defender makes their decision to pay the Fury Power before the Attacker.

The Sequence

According to Rule 442.1.b.1, when combat begins, triggered abilities are placed on the chain in a specific order: the Attacker places their triggers first, followed by the Defender.

1. Trigger Placement:
   • The Attacker's "When I attack" ability is placed on the chain first.
   • The Defender's "When I defend" ability is placed on the chain second (on top of the Attacker's ability).
2. Chain Resolution (LIFO - Last In, First Out):
   • Because the Defender's ability was added last, it is the first to resolve.
   • When the Defender's ability resolves, the Defender decides whether to pay the Fury Power.
   • After the Defender's ability finishes resolving, the Attacker's ability resolves, and the Attacker then decides whether to pay their Fury Power.

Key Nuance

As noted in the official FAQ for Draven, Vanquisher, the cost is paid on resolution, not when the ability is added to the chain. Because the Defender's ability is on top of the chain, they get to resolve their effect and make their decision before the Attacker's ability even begins to resolve.