The basic diagram above illustrates the observation that the higher the precipitant concentration, the lower the maximal achievable protein concentration (and vice versa). The metastable region (between solubility and decomposition) represents the supersaturated protein solution. This will eventually (MAYBE) equilibrate and separate into a protein-rich phase (in the form of precipitate or crystals) and a saturated protein solution. See diagram below.
This second diagram illustrated the principle of the vapor-diffusion, with the red arrow showing the path of a successful vapor-diffusion experiment. Once the region of spontaneous nucleation is reached, crystallization nuclei form (1) and a crystal may start to grow. The crystal will grow (2) in the "growth" region, as the solution depletes in protein. The final size of the crystal is reached when crystals are in equilibrium with the saturated protein solution (3).