Can you use Gust on Vayne as she moves to the battlefield before she becomes an attacker?

Ruling: You cannot Gust Vayne when she moves alone to an enemy-occupied battlefield because moving does not create a reaction opportunity and she instantly becomes an attacker with 5 might when combat begins. However, you can Gust her if she moves to an open battlefield (no combat occurs, so she's not an attacker) or if another unit with a move trigger creates a chain before the showdown begins.

Sequence:

• When combat begins, Assault and other passive abilities activate immediately
• An initial chain is built with all triggered abilities that care about combat beginning (including "When I attack/defend")
• Players can then add reactions to that chain before those abilities resolve

Nuances:

• Moving itself does not open a chain or create a reaction opportunity
• If another unit with a move trigger (like Traveling Merchant) moves alongside Vayne, that trigger opens a chain before the showdown, allowing you to Gust Vayne in that chain
• Vayne is only a valid Gust target when she's not an attacker (2 might or less), which only occurs when moving to an open battlefield or during a chain opened by another unit's move trigger