I don’t think Vader was cocky about the Death Star. He was openly dismissive of it in the briefing room scene. Vader did seem to better comprehend than Tarkin that smaller fighters were a bigger threat, when he decided to personally enter the battle.
He was dismissive of it in relation to the power of the Force, but not dismissive of its capabilities all together. But you’re right, Vader never seemed overly confident in the Death Star. What I meant to say was he was overly confident in being able to take down the Rebels, taking only 2 TIE fighters with him out to battle (both which ended up doing nothing if I remember because he insisted he would take care of them himself).
But this is getting way off topic so I think this conversation should end either here or very soon after.