For triggered abilities with optional costs (like Vayne's 'pay 1 energy to return me to hand'), when do you decide whether to pay the cost and actually pay it - when adding the trigger to the chain or on resolution?

Ruling: Costs formatted as "do X to do Y" within triggered ability instructions are paid on resolution, not when adding the trigger to the chain. You decide whether to pay the optional cost and pay it when the trigger resolves.

Sequence:

• The triggered ability is placed on the chain when its condition is met
• Other triggers and effects can be added to the chain and resolve first
• When the triggered ability resolves, you decide whether to pay the optional cost
• If you pay the cost, the effect happens (e.g., Vayne returns to hand)
• Opponents cannot react during resolution (i.e., while you're paying the cost and the effect is happening)

Nuances:

• These are called "costs within instructions" and are distinct from costs paid when playing/triggering abilities (which appear before the ':' in activated abilities)
• If multiple triggers occur simultaneously (like Vayne's trigger and another trigger from the same event), the owner can order them and resolve other triggers first before deciding whether to pay Vayne's optional cost