(And no - retribution and revenge are distinct concepts)
Perhaps they feel that a measure of justice in a society is whether the measure of punishment for a crime depends on the attrocity of the crime committed. Perhaps they feel that when a punished adult still gets nutritious meals, recreation, communication, perhaps even entertainment... while the raped/slaughtered victimes are still dead... that they do not live in a just society. And they celebrate justice being restored.
There are reasonable arguments for the contrary positios (that it isn't a deterent... or that victims' families do not always feel a sense of closure... or that taking a life cannot be justice in any real sense)... and no doubt you're preparted to argue that. But you're missing the point. You can't state that a given rationale is incorrect and then declare that it cannot therefore be their reason for "celebrating" - because their reasons for celebration do not need to match what you consider to be valid motivations in order to not be "revenge-driven" or just enjoying killing.
I place "celebrating" in quotes... because every such vigil that I'm aware of has been a very somber occasion. I've never seen (or seen reported) people having a "grand old time". Can you provide some examples?