Why water vapors exerts pressure rather than diffusing in air before reaching the boiling point of water?
I be mistaken, but are you referring to vapor pressure?

Vapor pressure  or equilibrium vapour pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of particles to escape from the liquid (or a solid). A substance with a high vapor pressure at normal temperatures is often referred to as volatile. (http://en.wikipedia.org/wiki/Vapor_pressure)

What is a closed system?

A system in which no mass is transferred.

Thus, vapor pressure would not be exerted if the container is open and the liquid is constantly diffusing. It would only occur in a closed system.

Therefore, liquids DO diffuse (evaporate) in air, rather than exert vapor pressure. 

You can leave a bottle of water, one with a lid and one without. The one without the lid will soon be without water, but the one with a lid would remain with water.

*Vapor pressure does relate to evaporation rate, which occurs in the open systems we normally observe in life