The Vanish Cap (replaced by a Vanish Flower in Super Mario Star Journey) is an item in Super Mario 64 and DS (in which only Luigi can use) that turns Mario into Vanish Mario, where he turns invisible and can travel though walls.
By putting on this cap, Mario, as Vanish Mario, becomes transparent and can walk though metal bars and just about all enemies.
In Super Fighting World's: Story mode, Smash Mode, Kombat Mode (in one player only) and Boss mode, Vanish caps appear. They make the user's opponents not be able to see the user for 20 seconds. In this, there are vanish caps of Mario, Luigi and Wario's caps. Any character can use any vanish cap though.