When is Vex considered 'in combat' during the sequence of an enemy merchant moving to attack at a battlefield?

Ruling: Vex enters combat after the merchant's move trigger chain fully resolves, starting with the Showdown's Initial Chain (attacker triggers). She exits combat after the resolution step concludes, which happens after clearing Contested status and after Conquer effects.

Sequence:

• Merchant move ability is added to the chain (combat is staged but not yet active)
• Both players react until the move chain fully resolves
• Combat begins with the Showdown, starting with the Initial Chain (attacker triggers)
• Defender triggers are added
• Both players react until fully resolved
• Action/reaction spell chains are played and resolved
• Damage is assigned
• Battlefield is no longer contested
• Conquer effects occur
• Combat ends after resolution step concludes

Nuances:

• Combat becomes "staged" when the merchant moves in, but you are not actually in combat until the move trigger chain fully resolves