Droplet levitation and coalescence mechanisms have consistently been a focal point in scientific research. From the perspective of molecular interactions, we investigated the influence of vapor pressure on the levitation and coalescence of droplets. The evaporation of nanodroplets and liquid pools under different initial environments was simulated by nonequilibrium molecular dynamics. The study analyzed the effects of evaporation during the processes of droplet levitation and coalescence and determined the existence of the Knudsen layer. Knudsen layers consisting of water molecules rather than clusters are generated during the evaporation. The definition, properties, and mechanisms of the layers were investigated.