Aether Vial counter-trigger question

[Soto]

This came up in an MWS game. My opponent had an Aether Vial out and I wanted to repeal it before he got a counter on it. I did so during his upkeep while the "you may put a counter on Aether Vial" was on the stack. My opponent said that was wrong, and the counter would have already been put on the Vial and he could sneak a creature in, in response to my Repeal. I couldn't find any convincing rulings in a quick search I did, so I'm posting here.

What is the correct ruling here?

Thanks

[Pinder]

Yes. Triggered abilities get put on the stack before they can resolve, so you have a chance to respond to them. Aether Vial has a triggered ability, so you can repeal it in response to the "put a counter on" trigger.