When Mimic Vat is in play and multiple creatures go to the graveyard at the same time, Mimic Vats’a ability will trigger once for each of these creatures. Technically these triggered abilities will on on the stack and resolve one at a time, an as each resolve the controller of the Mimic Vat will choose wether or not to remove the creature an Imprint it on the Vat. However it is a common shortcut to just state which creature, if any, you are going to imprint onto the Vat and resolve all of these triggers en masse.