For triggered abilities like Vayne - Hunter's that instruct you to pay energy, is the payment done at activation or at resolution of the ability?

Ruling: The payment is made at resolution of the triggered ability, not when it is placed on the chain.

Sequence:

• When the trigger condition is met, the triggered ability is placed on the chain
• Any targets are declared when the ability is placed on the chain
• When the ability resolves, the player decides whether to pay the cost (if optional) and the cost is paid
• The effect then occurs based on whether the cost was paid

Nuances:

• Costs within instructions (formatted as "Do X to Do Y" or "You may pay X to do Y") are paid at resolution, not when the ability goes on the chain
• This is different from "additional costs" which use the phrase "as an additional cost" and are paid when finalizing the card to the chain
• The "may" is part of the instruction itself, not a choice made before placing the ability on the chain
• Targets and other choices defined in rule 352 are still made when the ability is placed on the chain