If I charm my opponent's Volibear Furious into my battlefield and I have a Yuumi and a Vilemaw, would the Vilemaw still get +3 might and tank from Yuumi's effect or would she die to Volibear's effect first?

To determine the outcome of this interaction, we follow the Showdown Timing Flowchart and the rules regarding triggered abilities.

The Ruling

Vilemaw will receive the +3 Might and [Tank] from Yuumi, Magical Cat before Volibear, Furious deals his attack damage.

Sequence of Events

1. Initiation: Using Charm to move the opponent's Volibear, Furious to your battlefield initiates a combat showdown.
2. Phase 1 - Initial Chain:
   • Attacker's Triggers: As the attacker, Volibear's "When I attack" ability is added to the chain.
   • Defender's Triggers: As the defender, Yuumi's "When I defend" ability is added to the chain.
   • Chain Order: The chain is built (LIFO - Last In, First Out). Yuumi's ability is added last, placing it on top of Volibear's ability.
3. Resolution:
   • The chain resolves in reverse order. Yuumi's ability resolves first, granting Vilemaw +3 Might and [Tank].
   • Next, Volibear's ability resolves, dealing 5 damage split among your units.
4. Combat Damage Step:
   • After the chain is empty and all abilities have resolved, the game proceeds to the Combat Damage Step.
   • Because Vilemaw has [Tank], if you are assigned combat damage, you must assign lethal damage to Vilemaw first (Rule 815.1.b).

Key Nuances

• Triggered Ability Timing: According to the Showdown Timing Flowchart, "When I Attack" and "When I Defend" triggers are added to the chain before any player can play reactions. Because Yuumi's ability is a triggered ability, it uses the chain and resolves before combat damage is dealt (Rule 460.2).
• Tank: Vilemaw gains [Tank] from Yuumi's ability during the resolution of the Initial Chain. This status is active before the Combat Damage Step, ensuring Vilemaw is a valid target for damage assignment priority.
• Volibear's Damage: Volibear's ability damage is separate from combat damage (FAQ #8859). Even if Volibear's ability damages Vilemaw, Vilemaw remains on the board unless the damage is lethal (equal to or greater than its current Might).

Note: I could not fully verify this ruling against the retrieved rules.