Mirri's Guile & AEther Vial

[TobyornotToby]

When do you have to decide to put a counter on Aether Vial, when the ability triggers (or not) or when it resolves? In the second case I could stack it under the Mirri's Guile trigger to see the top 3 cards first before I decide whether to put a counter on Vial right?
Thanks =)

[heroicraptor]

Aether Vial's trigger has you decide whether or not to add a counter when the trigger resolves.

[dorsch]

When do you have to decide to put a counter on Aether Vial, when the ability triggers (or not) or when it resolves?
When the trigger resolves. Usually you do not make choices until resolution (except targets of course)

In the second case I could stack it under the Mirri's Guile trigger to see the top 3 cards first before I decide whether to put a counter on Vial right?
Thanks =)
Yes

[Sims]

Though question has been answered, I've done this a few times in Goblins when needing to replenish board state and get multiple warchief/chieftans on the table.

Vial on 3. EOT Vial in Chieftan. Untap, Upkeep put trigger on the stack, activate vial for warchief, then let counter trigger resolve and up to 4 so I can start vialing in ringleaders. Sure, now I can't ringleader at their end step, but it allows you to belch a few lords on the table for a fast recovery/alpha strike while not missing your vial triggers.

You're basically doing the same thing, except you're putting the trigger on the stack, then guile. Guile resolves and lets you look, then you can either let the trigger resolve with no change, or up it to take advantage of the creature on top of your library.