Vapour pressure and change in volume

The volume is proportional to the molecules of a gas. If the volume is increase, so the number of the molecules of the gas is increase too and vice versa: if the volume is decrease, so the number of the molecules of the gas is decrease too. That's why the vapour pressure and the volume are constant:

p * V = constant.

In other words: the vapour pressure is always the same.

Thanks!

Why doesn't change in volume affect vapour pressure?

As asked this is wrong.

It is correct for a SATURATED vapor. In such situation when you change the volume (increasing the pressure!), some of the vapor will condense, and the pressure will go down back to the initial value. If you increase the volume AND there is enough liquid present, pressure will initially decrease, but soon some of the liquid will evaporate and the pressure will go up back to the initial value. If you will change the volume very slowly, pressure will look constant (condensation/evaporation will be fast enough).

If the vapor is not saturated decreasing volume will increase the pressure, if there is no liquid present increasing the volume will decrease the pressure,so the question as worded will make no sense.