When an opponent's creature with undying dies, both the Mimic Vat and the Undying trigger will go on the stack, and which one resolves first depends on who's turn it is.
But what if the opponent is not aware of how this works exactly?
If it's my turn, and my opponent's undying creature dies, and I state “Mimic Vat trigger” and my opponent hands me the creature to put under it, have I committed an infraction? Under Game Play Error — Missed Trigger it says:
Players may not cause triggered abilities controlled by an opponent to be missed by taking game actions or otherwise prematurely advancing the game.
But I wasn't the one advancing the game. I just stated that a trigger would go on the stack. Can him handing me the creature be interpreted as him not resolving the undying trigger and allowing me to imprint the creature on Mimic Vat?