Why does atmospheric pressure reduce as water vapour in air increases?

Why does atmospheric pressure reduce as water vapour in air increases?

Water vapour is lighter than air, that's why atmospheric pressure reduces as water vapor in air increases.

Water vapour goes up while cooking. From this, we can assume, water vapour's density is less than air. That's why, as water vapor increases, the average density of air decreases. So, atmospheric pressure reduces as water vapor in air increases.