When Midnight Arsonist enters the battlefield, destroy up to X target artifacts without mana abilities, where X is the number of Vampires you control.
Midnight Arsonist's triggered ability can't target artifacts that have either activated or triggered mana abilities. An activated mana ability is one that could add mana to a player's mana pool as it resolves, doesn't have a target, and isn't a loyalty ability. A triggered mana ability is one that adds mana to a player's mana pool and triggers based on an activated mana ability.