When we add salt to a vessel containing water, the surface area of water molecules decrease as some of them are replaced by salt molecules. Due to the less availability of water molecules on the surface, the vapour pressure decreases.

Also lowering the temperature of water will lower the kinetic energy of the water molecules, the randomness of the water molecules will also decrease on the surface so gradually the vapour pressure also decreases.

Decreasing the quantity of water or the volume of the vessel has no effect on the vapour pressure because no change is done to the orientation of these water molecules.