Q:I have a question regarding Captivating Vampire. So let's say my opponent has his 5 vampires out on the battlefield. He taps the 4 vampires and Captivating Vampire to gain control of one of my creatures. I reply with Grasp of Darkness on his CVampire. But he says that he still gains control of my creature. Is this true?
No, as tapping the five vampires is a cost and costs can’t be responded to. You can cast Grasp after the Captivating’s activation, but its ability will still go through.