Vapour-liquid equilibrium (VLE) is defined by the condition where a liquid and its vapour phase are in equilibrium with each other. Under this state or condition, there is no net change in vapour-liquid phases thus the rate of evaporation (liquid to vapour) is equal to the rate of condensation (vapour to liquid). In theory, the state of equilibrium would take forever to be reached but such equilibrium can be practically achieved within a closed system where the liquid and vapour are allowed to stand in contact with each other long enough with literally no interference or only slight interference from the exteriors.