Thermodynamics: Vapor Liquid Equilibrium

When we say the vapors are in equilibrium with the liquid, what property of the two phases will be equal? Whether it is temperature, pressure, concentration or something else that will be same? 
Equilibrium stands for no net driving force, so what is the driving force for vapor liquid system?
It is chemical potential or fugacity that is same in two phases.

Suppose we have a liquid mixture of ethyl alcohol (NBP 78.5°C) and water (NBP 100°C). At any temperature, there will be some amount of alcohol & water vapors in contact with the liquid phase. Ethyl alcohol is more volatile as compared to water, so ethyl alcohol has more concentration in the vapor phase and but how it will be calculated?

The amount of alcohol & water in vapor phase can be calculated by their vapor pressure at that temperature. The vapor pressure of alcohol is always greater than that of water. For example at 80°C the vapor pressure of alcohol is 1 atm while that of water is less, which will be 1 atm at 100°C. Therefore, at equilibrium: for ideal system i.e. ideal gas phase and ideal liquid phase (Raoult’s law):
P_Ty_A = Pv_Ax_A;  

Pv_{A is the vapor pressure of component A at specified temperature. yA is the mole fraction of component A in vapor phase and} xA _{is the mole fraction of component A} in liquid.

If partial pressure of component A (P_Ty_A) is less in vapor phase than corresponding to liquid phase (Pv_Ax_A), evaporation continues (increases y) till both are equal. and if partial pressure is greater than _{in vapor phase than corresponding to liquid phase,} condensation will start and continues (decreases y) till both are equal. 

So condensation can be made to happen by either increasing total pressure (P_T) or decreasing temperature which decreases the vapor pressure (PA)